Semiconductor device and method for manufacturing the same

ABSTRACT

A semiconductor device includes a substrate having an insulating surface; a light-transmitting first electrode provided over the substrate; a light-transmitting second electrode provided over the substrate; a light-transmitting semiconductor layer provided so as to be electrically connected to the first electrode and the second electrode; a first wiring electrically connected to the first electrode; an insulating layer provided so as to cover at least the semiconductor layer; a light-transmitting third electrode provided over the insulating layer in a region overlapping with the semiconductor layer; and a second wiring electrically connected to the third electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The technical field relates to a semiconductor device, a display device,a light-emitting device, and a method for manufacturing these devices.The technical field particularly relates to a semiconductor deviceincluding a thin film transistor (hereinafter also referred to as a TFT)using an oxide semiconductor.

2. Description of the Related Art

In recent years, thin film transistors (TFTs) in which a silicon layerof amorphous silicon or the like is used as a channel layer have beenwidely used as switching elements in display devices typified by liquidcrystal display devices. Although the field-effect mobility is low, athin film transistor using amorphous silicon has an advantage inresponding to increase in size of glass substrate.

Moreover, attention has been recently drawn to a technique by which athin film transistor is manufactured using a metal oxide withsemiconductor characteristics and such a transistor is applied to anelectronic device or an optical device. For example, it is known thatsome metal oxides such as tungsten oxide, tin oxide, indium oxide, andzinc oxide have semiconductor characteristics. A thin film transistor inwhich a transparent semiconductor layer formed of such a metal oxide isused as a channel formation region is disclosed (e.g., see PatentDocument 1).

Furthermore, a technique has been considered to increase the apertureratio in such a manner that a channel layer of a transistor is formed ofa light-transmitting oxide semiconductor layer and a gate electrode, asource electrode, and a drain electrode are formed of a transparentconductive film with a light-transmitting property (e.g., see PatentDocument 2).

Increase in aperture ratio increases the light use efficiency, andreduction in power and size of a display device can be achieved.Meanwhile, in terms of increase in size and application to portabledevices of display devices, further reduction in power consumption aswell as increase in aperture ratio is required.

As a method for placing a metal auxiliary wiring for a transparentelectrode of an electro-optic element, a method is known in which ametal auxiliary wiring and a transparent electrode are placed to overlapwith each other so that the auxiliary wiring is brought into conductionwith the transparent electrode above or below the transparent electrode(e.g., see Patent Document 3).

A structure is known in which an additional capacitance electrodeprovided for an active matrix substrate is formed of a transparentconductive film of ITO, SnO₂, or the like and an auxiliary wiring formedof a metal film is provided in contact with the additional capacitanceelectrode in order to reduce the electric resistance of the additionalcapacitance electrode (e.g., see Patent Document 4).

It is known that in an electric-field transistor using an amorphousoxide semiconductor film, a transparent electrode formed of indium tinoxide (ITO), indium zinc oxide, ZnO, SnO₂, or the like; a metalelectrode formed of Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like; a metalelectrode formed of an alloy containing any of the above elements; orthe like can be used for a gate electrode, a source electrode, and adrain electrode, and two or more of such materials may be stacked toreduce the contact resistance or to increase the interface intensity(e.g., see Patent Document 5).

It is known that a metal such as indium (In), aluminum (Al), gold (Au),or silver (Ag); or an oxide material such as indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmiumoxide (CdIn₂O₄), cadmium tin oxide (Cd₂SnO₄), or zinc tin oxide(Zn₂SnO₄) can be used as materials for a source electrode, a drainelectrode, and a gate electrode of a transistor using an amorphous oxidesemiconductor and an auxiliary capacitance electrode, and the materialsfor the gate electrode, the source electrode, and the drain electrodemay be the same or different from each other (e.g., see Patent Documents6 and 7).

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2004-103957-   Patent Document 2: Japanese Published Patent Application No.    2007-081362-   Patent Document 3: Japanese Published Patent Application No.    H2-082221-   Patent Document 4: Japanese Published Patent Application No.    H2-310536-   Patent Document 5: Japanese Published Patent Application No.    2008-243928-   Patent Document 6: Japanese Published Patent Application No.    2007-109918-   Patent Document 7: Japanese Published Patent Application No.    2007-115807

SUMMARY OF THE INVENTION

In view of the above, an object of one embodiment of the inventiondisclosed in this specification and the like (at least including thespecification, the scope of claims, and the drawings) is to provide asemiconductor device with high aperture ratio, to provide asemiconductor device with low power consumption, to provide asemiconductor device with low wiring resistance, to provide asemiconductor device in which distortion of signal waveforms is reduced,to provide a wiring with high conductivity, to provide a semiconductordevice with high transmittance, to provide a semiconductor device havinga large screen, to provide a semiconductor device in which increase inthe number of steps for a process is suppressed, to provide asemiconductor device with high contrast, to provide a semiconductordevice with high layout flexibility, or to provide a semiconductordevice with low subthreshold swing value. Note that these objects do notdeny the existence of other objects. Further, one embodiment of thedisclosed invention is not necessary to achieve all the above objects.

In one embodiment of the invention disclosed in this specification andthe like, a transistor is formed using a light-transmitting material.Further details are as follows.

One embodiment of the invention disclosed in this specification and thelike is a semiconductor device including a substrate having aninsulating surface; a first electrode (a source electrode) that has alight-transmitting property and is provided over the substrate; a secondelectrode (a drain electrode) that has a light-transmitting property andis provided over the substrate; a semiconductor layer that has alight-transmitting property and is provided so as to be electricallyconnected to the first electrode and the second electrode; a firstwiring (a source wiring) electrically connected to the first electrode;an insulating layer (a gate insulating layer) provided so as to cover atleast the semiconductor layer; a third electrode (a gate electrode) thathas a light-transmitting property and is provided over the insulatinglayer in a region overlapping with the semiconductor layer; and a secondwiring (a gate wiring) electrically connected to the third electrode.

Another embodiment of the invention disclosed in this specification andthe like is a method for manufacturing a semiconductor device, includingthe steps of: stacking a first conductive layer having alight-transmitting property and a second conductive layer over asubstrate having an insulating surface; forming a first mask over thesecond conductive layer; etching the first conductive layer to form afirst electrode and a second electrode and etching the second conductivelayer to form a third conductive layer, by using the first mask;recessing the first mask to form a second mask; etching the thirdconductive layer by using the second mask to form a first wiring;forming a semiconductor layer that has a light-transmitting property andis electrically connected to the first electrode and the secondelectrode; forming an insulating layer so as to cover the semiconductorlayer; stacking a fourth conductive layer having a light-transmittingproperty and a fifth conductive layer over the insulating layer; forminga third mask over the fifth conductive layer; etching the fourthconductive layer to form a third electrode and etching the fifthconductive layer to form a sixth conductive layer, by using the thirdmask; recessing the third mask to form a fourth mask; and etching thesixth conductive layer by using the fourth mask to form a second wiring.

Note that in the above, a fourth electrode (a pixel electrode) that hasa light-transmitting property and is electrically connected to thesecond electrode may be provided. Moreover, a fifth electrode (acapacitor electrode) which is provided in a region overlapping with partof the second electrode with the insulating layer therebetween and isformed by using the same layer as the third electrode; and a thirdwiring (a capacitor wiring) which is electrically connected to the fifthelectrode and is formed by using the same layer as the second wiring maybe provided.

In addition, in the above, the semiconductor layer is preferably formedusing an oxide semiconductor containing indium, gallium, and zinc.Further, each of the first electrode, the second electrode, and thethird electrode is preferably formed using any of indium tin oxide,indium tin oxide containing silicon oxide, organoindium, organotin, zincoxide, titanium nitride, indium zinc oxide containing zinc oxide, amaterial obtained by adding gallium to zinc oxide, tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, and indium tin oxidecontaining titanium oxide.

Note that in the above, the first wiring and the second wiringpreferably have a light-shielding property. Moreover, a layer formed byusing the same layer as the semiconductor layer is provided at anintersection of the first wiring and the second wiring. Accordingly, thecapacitance generated because wirings intersect each other can bereduced, so that distortion of signal waveforms can be suppressed. Thisis particularly effective in large semiconductor devices.

Note that an example of an oxide semiconductor which can be used in theinvention disclosed in this specification and the like is an oxidesemiconductor represented by InMO₃ (ZnO)_(m) (m>0). Here, M denotes oneor more of metal elements selected from gallium (Ga), iron (Fe), nickel(Ni), manganese (Mn), and cobalt (Co). For example, the case where Ga isselected as M includes the case where only Ga is used and the case wherethe above metal element other than Ga is contained in addition to Ga,for example, a combination of Ga and Ni or a combination of Ga and Fe isused. Moreover, in the above oxide semiconductor, in some cases, atransition metal element such as Fe or Ni or an oxide of the transitionmetal is contained as an impurity element in addition to a metal elementcontained as M. In this specification and the like, among the aboveoxide semiconductors, an oxide semiconductor including at least galliumas M is referred to as an In—Ga—Zn—O-based oxide semiconductor and athin film using the material is referred to as an In—Ga—Zn—O-basednon-single-crystal film in some cases.

Further, in the above, by using a multi-tone mask, a light-transmittingregion (a region with high transmittance) and a region without alight-transmitting property (a region with low transmittance) can beformed with one mask (reticle). Thus, increase in the number of maskscan be suppressed.

Note that a semiconductor device in this specification and the likeindicates general devices capable of functioning by utilizingsemiconductor characteristics. Semiconductor circuits, display devices,electro-optic devices, light-emitting display devices, and electronicdevices are all included in the category of the semiconductor device.

In addition, a display device in this specification and the like refersto an image display device, a light-emitting device, or a light source(including a lighting device). Further, the display device also includesthe following modules in its category: a module to which a connectorsuch as an FPC (flexible printed circuit), a TAB (tape automatedbonding) tape, or a TCP (tape carrier package) is attached; a moduleprovided with a printed wiring board at an end of a TAB tape or a TCP; amodule in which an integrated circuit (IC) is directly mounted on adisplay element by a COG (chip on glass) method, and the like.

Note that various types of switches can be used as a switch. Examplesare an electrical switch and a mechanical switch. That is, there is noparticular limitation on the kind of switch as long as it can controlthe flow of current. For example, a transistor (e.g., a bipolartransistor or a MOS transistor), a diode (e.g., a PN diode, a PIN diode,a Schottky diode, a metal-insulator-metal (MIM) diode, ametal-insulator-semiconductor (MIS) diode, or a diode-connectedtransistor), or the like can be used as a switch. Alternatively, a logiccircuit combining such elements can be used as a switch.

As examples of mechanical switches, there is a switch formed by a microelectro mechanical system (MEMS) technology, such as a digitalmicromirror device (DMD). Such a switch includes an electrode which canbe moved mechanically, and operates to control electrical connection ornon-electrical-connection with the movement of the electrode.

When a transistor is used as a switch, the polarity (conductivity type)of the transistor is not particularly limited because it operates as amere switch. Note that a transistor of polarity with smaller off-statecurrent is preferably used when the off-state current should be small.Examples of a transistor with smaller off-state current are a transistorprovided with an LDD region and a transistor with a multi-gatestructure. Further, an n-channel transistor is preferably used when thetransistor operates with a potential of a source terminal closer to apotential of a low potential side power supply (e.g., Vss, GND, or 0 V).On the other hand, a p-channel transistor is preferably used when thetransistor operates with a potential of a source terminal close to apotential of a high potential side power supply (e.g., Vdd). This isbecause when the n-channel transistor operates with the potential of thesource terminal close to a low potential side power supply or thep-channel transistor operates with the potential of the source terminalclose to a high potential side power supply, an absolute value of agate-source voltage can be increased; thus, the transistor can moreprecisely operate as a switch. Moreover, this is because reduction inoutput voltage does not often occur because the transistor does notoften perform a source follower operation.

Note that a CMOS switch may be employed as a switch by using bothn-channel and p-channel transistors. By employing a CMOS switch, thetransistor can more precisely operate as a switch because a current canflow when either the p-channel transistor or the n-channel transistor isturned on. For example, even when a voltage of an input signal to aswitch is high or low, an appropriate voltage can be output. Further,since a voltage amplitude value of a signal for turning on or off theswitch can be made small, power consumption can be reduced.

Note that when a transistor is employed as a switch, the switch includesan input terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal for controlling electrical conduction (a gateterminal). On the other hand, when a diode is employed as a switch, theswitch does not have a terminal for controlling electrical conduction insome cases. Therefore, when a diode is used as a switch, the number ofwirings for controlling terminals can be reduced as compared to the caseof using a transistor.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, another elementmay be provided between elements having a connection relationillustrated in drawings and texts, without limitation on a predeterminedconnection relation, for example, the connection relation illustrated inthe drawings and the texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection between A and B(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,and/or a diode) may be connected between A and B. In the case where Aand B are functionally connected, one or more circuits which enablefunctional connection between A and B (e.g., a logic circuit such as aninverter, a NAND circuit, or a NOR circuit; a signal converter circuitsuch as a DA converter circuit, an AD converter circuit, or a gammacorrection circuit; a potential level converter circuit such as a powersupply circuit (e.g., a dc-dc converter, a step-up dc-dc converter, or astep-down dc-dc converter) or a level shifter circuit for changing apotential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit which canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; and/or a control circuit) may be connected between A andB. For example, in the case where a signal output from A is transmittedto B even when another circuit is provided between A and B, A and B arefunctionally connected.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected with another element or another circuittherebetween), the case where A and B are functionally connected (i.e.,the case where A and B are functionally connected with another circuittherebetween), and the case where A and B are directly connected (i.e.,the case where A and B are connected without another element or anothercircuit therebetween) are included therein. That is, when it isexplicitly described that “A and B are electrically connected”, thedescription is the same as the case where it is explicitly onlydescribed that “A and B are connected”.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ a variety of modes and include a variety of elements.For example, a display element, a display device, a light-emittingelement, and a light-emitting device can include a display medium whosecontrast, luminance, reflectivity, transmittance, or the like changes byelectromagnetic action, such as an EL (electroluminescence) element(e.g., an EL element containing organic and inorganic materials, anorganic EL element, or an inorganic EL element), an LED (e.g., a whiteLED, a red LED, a green LED, or a blue LED), a transistor (a transistorwhich emits light depending on the amount of current), an electronemitter, a liquid crystal element, electronic ink, an electrophoreticelement, a grating light valve (GLV), a plasma display panel (PDP), adigital micromirror device (DMD), a piezoelectric ceramic display, or acarbon nanotube. Note that examples of display devices using an ELelement are an EL display; examples of display devices using an electronemitter are a field emission display (FED) and an SED(surface-conduction electron-emitter display) flat panel display;examples of display devices using a liquid crystal element are a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, and a projection liquidcrystal display); and examples of display devices using electronic inkor an electrophoretic element are electronic paper.

An EL element is an element including an anode, a cathode, and an ELlayer placed between the anode and the cathode. The EL layer can be, forexample, a layer utilizing emission from a singlet exciton(fluorescence) or a triplet exciton (phosphorescence), a layer utilizingemission from a singlet exciton (fluorescence) and emission from atriplet exciton (phosphorescence), a layer containing an organicmaterial or an inorganic material, a layer containing an organicmaterial and an inorganic material, a layer containing a high molecularmaterial or a low molecular material, and a layer containing a lowmolecular material and a high molecular material. Note that the ELelement can include a variety of layers without limitation to thosedescribed above.

An electron emitter is an element in which electrons are extracted byhigh electric field concentration on a cathode. For example, theelectron emitter can be any one of a Spindt-type, a carbon nanotube(CNT) type, a metal-insulator-metal (MIM) type including a stack of ametal, an insulator, and a metal, a metal-insulator-semiconductor (MIS)type including a stack of a metal, an insulator, and a semiconductor, aMOS type, a silicon type, a thin film diode type, a diamond type, asurface conductive emitter SCD type, a thin film type in which a metal,an insulator, a semiconductor, and a metal are stacked, a HEED type, anEL type, a porous silicon type, a surface-conduction electron-emitter(SCD) type, and the like. Note that various elements can be used as anelectron emitter without limitation to those described above.

A liquid crystal element is an element that controls transmission ornon-transmission of light by an optical modulation action of liquidcrystal, and includes a pair of electrodes and liquid crystal. Theoptical modulation action of liquid crystal is controlled by an electricfield (including a lateral electric field, a vertical electric field,and a diagonal electric field) applied to the liquid crystal. Thefollowing liquid crystal can be used for a liquid crystal element:nematic liquid crystal, cholesteric liquid crystal, smectic liquidcrystal, discotic liquid crystal, thermotropic liquid crystal, lyotropicliquid crystal, low molecular liquid crystal, high molecular liquidcrystal, polymer dispersed liquid crystal (PDLC), ferroelectric liquidcrystal, anti-ferroelectric liquid crystal, main chain type liquidcrystal, side chain type polymer liquid crystal, plasma addressed liquidcrystal (PALC), and banana-shaped liquid crystal. Moreover, thefollowing methods can be used for driving the liquid crystal, forexample: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV (advanced super view) mode, an ASM(axially symmetric aligned microcell) mode, an OCB (opticallycompensated birefringence) mode, an ECB (electrically controlledbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a guest-host mode, and a blue phase mode. Notethat various kinds of liquid crystal elements and driving methods can beused without limitation on those described above.

Electronic paper corresponds to devices that display images by moleculeswhich utilize optical anisotropy, dye molecular orientation, or thelike; by particles which utilize electrophoresis, particle movement,particle rotation, phase change, or the like; by moving one end of afilm; by using coloring properties or phase change of molecules; byusing optical absorption by molecules; and by using self-light emissionby combining electrons and holes. For example, the following can be usedfor the electronic paper: microcapsule electrophoresis, horizontalelectrophoresis, vertical electrophoresis, a spherical twisting ball, amagnetic twisting ball, a columnar twisting ball, a charged toner,electro liquid powder, magnetic electrophoresis, a magneticthermosensitive type, an electrowetting type, a light-scattering(transparent-opaque change) type, cholesteric liquid crystal and aphotoconductive layer, a cholesteric liquid crystal device, bistablenematic liquid crystal, ferroelectric liquid crystal, a liquid crystaldispersed type with a dichroic dye, a movable film, coloring anddecoloring properties of a leuco dye, a photochromic material, anelectrochromic material, an electrodeposition material, flexible organicEL, and the like. Note that various types of electronic papers can beused without limitation to those described above. By using microcapsuleelectrophoresis, problems of electrophoresis, that is, aggregation orprecipitation of electrophoretic particles can be solved. Electro liquidpowder has advantages such as high-speed response, high reflectivity,wide viewing angle, low power consumption, and memory properties.

A plasma display panel includes a substrate having a surface providedwith an electrode, and a substrate having a surface provided with anelectrode and a minute groove in which a phosphor layer is formed. Inthe plasma display panel, the substrates are opposite to each other witha narrow interval, and a rare gas is sealed therein. Alternatively, aplasma display panel can have a structure in which a plasma tube isplaced between film-shaped electrodes. A plasma tube is such that adischarge gas, fluorescent materials for RGB, and the like are sealed ina glass tube. Display can be performed by applying a voltage between theelectrodes to generate an ultraviolet ray so that the fluorescentmaterials emit light. Note that the plasma display panel may be a DCtype PDP or an AC type PDP. As a method for driving the plasma displaypanel, AWS (address while sustain) driving, ADS (address displayseparated) driving in which a subframe is divided into a reset period,an address period, and a sustain period, CLEAR (high-contrast, lowenergy address and reduction of false contour sequence) driving, ALIS(alternate lighting of surfaces) method, TERES (technology of reciprocalsustainer) driving, and the like can be used. Note that various methodfor driving a plasma display panel can be used without limitation tothose described above.

Electroluminescence, a cold cathode fluorescent lamp, a hot cathodefluorescent lamp, an LED, a laser light source, a mercury lamp, or thelike can be used for a light source of a display device in which a lightsource is needed, such as a liquid crystal display device (atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, and a projection type liquid crystal display), adisplay device using a grating light valve (GLV), and a display deviceusing a digital micromirror device (DMD). Note that a variety of lightsources can be used without limitation to those described above.

Note that as a transistor, various types of transistors can be usedwithout being limited to a certain type. For example, a thin filmtransistor (TFT) including a non-single-crystal semiconductor filmtypified by amorphous silicon, polycrystalline silicon, microcrystalline(also referred to as microcrystal, nanocrystal, or semi-amorphous)silicon, or the like can be used. The use of such TFTs has variousadvantages. For example, since TFTs can be formed at temperature lowerthan those using single crystalline silicon, manufacturing costs can bereduced or a manufacturing device can be made larger. Since themanufacturing device can be made larger, the TFTs can be formed using alarge substrate. Therefore, a plurality of display devices can be formedat the same time, so that manufacturing costs can be reduced. Inaddition, since the manufacturing temperature is low, a substrate havinglow heat resistance can be used. Accordingly, the transistor can beformed over a light-transmitting substrate such as a glass substrate.Moreover, transmission of light in a display element can be controlledby using the transistors formed using the light-transmitting substrate.Further, part of a film included in the transistor can transmit lightbecause the transistor is thin. Accordingly, the aperture ratio can beincreased.

When polycrystalline silicon is formed, the use of a catalyst (e.g.,nickel) enables further improvement in crystallinity and formation of atransistor with excellent electrical characteristics. Accordingly, agate driver circuit (a scan line driver circuit), a source drivercircuit (a signal line driver circuit), and a signal processing circuit(e.g., a signal generation circuit, a gamma correction circuit, or a DAconverter circuit) can be formed over one substrate.

In addition, when microcrystalline silicon is formed, the use of acatalyst (e.g., nickel) enables further improvement in crystallinity andformation of a transistor with excellent electrical characteristics. Atthis time, the crystallinity can be improved by applying heat withoutlaser irradiation. Thus, part of a source driver circuit (e.g., ananalog switch) and a gate driver circuit (a scan line driver circuit)can be formed over one substrate. Further, when laser irradiation forcrystallization is not performed, unevenness of silicon crystallinitycan be suppressed. Accordingly, images with improved quality can bedisplayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without using a catalyst (e.g., nickel).

The crystallinity of silicon is preferably enhanced to polycrystallinityor microcrystallinity in the entire panel, but not limited thereto. Thecrystallinity of silicon may be increased only in part of the panel. Theselective increase in crystallinity can be achieved by selective laserirradiation or the like. For example, only a peripheral driver circuitregion excluding pixels may be irradiated with laser light.Alternatively, only a region of a gate driver circuit, a source drivercircuit, or the like may be irradiated with laser light. Furtheralternatively, only part of a source driver circuit (e.g., an analogswitch) may be irradiated with laser light. Accordingly, thecrystallinity of silicon only in a region in which a circuit needs tooperate at high speed can be increased. Since a pixel region does notespecially need to operate at high speed, the pixel circuit can operatewithout problems even if the crystallinity is not increased. A regionwhose crystallinity is increased is small, whereby manufacturing stepscan be reduced, the throughput can be increased, and manufacturing costscan be reduced. Since the number of manufacturing devices needed issmall, it is possible to reduce (not to increase) manufacturing costs.

A transistor can be formed using a semiconductor substrate, an SOIsubstrate, or the like. Accordingly, a transistor with few variations incharacteristics, sizes, shapes, or the like, with high current supplycapability, and with a small size can be formed. By using suchtransistors, power consumption of a circuit can be reduced or a circuitcan be highly integrated.

Alternatively, a transistor including a compound semiconductor or anoxide semiconductor, such as ZnO, a-InGaZnO, IZO, ITO, SnO, TiO, AlZnSnO(AZTO), and a thin film transistor or the like obtained by thinning sucha compound semiconductor or oxide semiconductor can be used.Accordingly, the manufacturing temperature can be lowered and forexample, such a transistor can be formed at room temperature. Thus, thetransistor can be formed directly on a substrate having low heatresistance, such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or oxide semiconductor can be used for notonly a channel portion of a transistor but also for other applications.For example, such a compound semiconductor or oxide semiconductor can beused for a resistor, a pixel electrode, or a light-transmittingelectrode. Further, since such an element can be formed at the same timeas the transistor, the costs can be reduced. Alternatively, asemiconductor such as SiGe or GaAs may be used.

Transistors or the like formed by an inkjet method or a printing methodcan also be used. Accordingly, transistors can be formed at roomtemperature, can be formed at a low vacuum, or can be formed using alarge substrate. Since such transistors can be formed without a mask (areticle), the layout of the transistors can be easily changed. Moreover,since it is not necessary to use a resist, the material costs arereduced and the number of steps can be reduced. Further, since a film isformed where needed, the material is not wasted compared to amanufacturing method in which etching is performed after a film isformed over the entire surface, so that the costs can be reduced.

Further, transistors or the like including an organic semiconductor or acarbon nanotube can be used. Such transistors can be formed over aflexible substrate. A semiconductor device using such a substrate canresist a shock.

In addition, various types of transistors can be used. For example, aMOS transistor, a junction transistor, a bipolar transistor, or the likecan be employed. Since a MOS transistor has a small size, a large numberof transistors can be mounted. The use of a bipolar transistor can allowa large current to flow; thus, a circuit can operate at high speed.

Further, a MOS transistor, a bipolar transistor, and the like may beformed over one substrate. Thus, low power consumption, reduction insize, and high-speed operation can be achieved.

Furthermore, various transistors other than the above transistors can beused.

A transistor can be formed using various types of substrates. The typeof a substrate is not limited to a certain type. As the substrate, asingle crystalline substrate (e.g., a silicon substrate), an SOIsubstrate, a glass substrate, a quartz substrate, a plastic substrate, ametal substrate, a stainless steel substrate, a substrate including astainless steel foil, a tungsten substrate, a substrate including atungsten foil, or a flexible substrate can be used, for example.Examples of the glass substrate are barium borosilicate glass andaluminoborosilicate glass. Examples of the flexible substrate areflexible synthetic resin such as plastics typified by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), andpolyethersulfone (PES), and acrylic. Alternatively, an attachment film(formed using polypropylene, polyester, vinyl, polyvinyl fluoride,polyvinyl chloride, or the like), paper including a fibrous material, abase material film (polyester, polyamide, polyimide, an inorganic vapordeposition film, paper, or the like), or the like can be used.Alternatively, the transistor may be formed using one substrate, andthen, the transistor may be transferred to another substrate. As asubstrate to which the transistor is transferred, a single crystalsubstrate, an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a paper substrate, a cellophane substrate, a stonesubstrate, a wood substrate, a cloth substrate (including a naturalfiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon,polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra,rayon, or regenerated polyester), or the like), a leather substrate, arubber substrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used. A skin (e.g., epidermisor corium) or hypodermal tissue of an animal such as a human being maybe used as a substrate to which the transistor is transferred.Alternatively, the transistor may be formed using one substrate and thesubstrate may be thinned by polishing. As a substrate to be polished, asingle crystal substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a stainless steel substrate, a substrateincluding a stainless steel foil, or the like can be used. By using sucha substrate, a transistor with excellent properties or low powerconsumption can be formed, a device with high durability or high heatresistance can be provided, or reduction in weight or thickness can beachieved.

Note that a structure of a transistor can employ various modes withoutbeing limited to a specific structure. For example, a multi-gatestructure having two or more gate electrodes can be used. When themulti-gate structure is used, a structure where a plurality oftransistors are connected in series is provided because channel regionsare connected in series. With the multi-gate structure, the off-statecurrent can be reduced and the withstand voltage of the transistor canbe increased (the reliability can be increased). Further, by employingthe multi-gate structure, a drain-source current does not change mucheven if a drain-source voltage changes when the transistor operates in asaturation region; thus, the slope of voltage-current characteristicscan be flat. By utilizing the characteristics that the slope of thevoltage-current characteristics is flat, an ideal current source circuitor an active load having an extremely high resistance value can beprovided. Accordingly, a differential circuit or a current mirrorcircuit which has excellent properties can be provided.

As another example, a structure where gate electrodes are formed aboveand below a channel can be used. By employing the structure where gateelectrodes are formed above and below the channel, a channel region isenlarged; thus, a current value can be increased. Alternatively, byemploying the structure where gate electrodes are formed above and belowthe channel, a depletion layer is easily formed, so that subthresholdswing (S value) can be improved. When the gate electrodes are formedabove and below the channel, a structure where a plurality oftransistors are connected in parallel is provided.

Further, a structure where a gate electrode is formed above or below achannel, a staggered structure, an inverted staggered structure, astructure where a channel region is divided into a plurality of regions,a structure where channel regions are connected in parallel or in seriescan also be employed. In addition, a source electrode or a drainelectrode may overlap with a channel region (or part of it). By usingthe structure where the source electrode or the drain electrode mayoverlap with the channel region (or part of it), unstable operation dueto electric charge accumulated in part of the channel region can beprevented. Further, an LDD region can be provided. By providing the LDDregion, the off-state current can be reduced or the withstand voltage ofthe transistor can be increased (the reliability can be increased).Alternatively, by providing the LDD region, a drain-source current doesnot change much even if a drain-source voltage changes when a transistoroperates in the saturation region, so that a slope of voltage-currentcharacteristics can be flat.

Note that a variety of transistors can be used, and the transistor canbe formed using a variety of substrates. Accordingly, all the circuitswhich are necessary to realize a predetermined function can be formedusing one substrate. For example, all the circuits which are necessaryto realize the predetermined function can be formed using a glasssubstrate, a plastic substrate, a single crystal substrate, an SOIsubstrate, or any other substrate. When all of the circuits which arenecessary to realize the predetermined function are formed using onesubstrate, the number of components can be reduced to cut the costs orthe number of connections between circuit components can be reduced toincrease the reliability. Alternatively, some of the circuits which arenecessary to realize the predetermined function can be formed using onesubstrate and other circuits which are necessary to realize thepredetermined function can be formed using another substrate. That is,not all the circuits which are necessary to realize the predeterminedfunction need to be formed using one substrate. For example, some of thecircuits which are necessary to realize the predetermined function canbe formed by transistors using a glass substrate, other circuits whichare necessary to realize the predetermined function can be formed usinga single crystal substrate, and an IC chip including transistors formedusing the single crystal substrate can be connected to the glasssubstrate by COG (chip on glass) so that the IC chip is provided overthe glass substrate. Alternatively, the IC chip can be connected to theglass substrate by TAB (tape automated bonding) or a printed wiringboard. When part of the circuits are formed using the same substrate insuch a manner, the number of the components can be reduced to cut thecosts or the number of connections between the circuit components can bereduced to increase the reliability. In addition, circuits in a portionwith high driving voltage or a portion with high driving frequencyconsume large power. Accordingly, when the circuits in such portions areformed using a single crystalline substrate, for example, instead ofusing the same substrate, and an IC chip formed by the circuit is used,increase in power consumption can be prevented.

Note that one pixel corresponds to one element whose brightness can becontrolled. For example, one pixel corresponds to one color element, andbrightness is expressed with one color element. Accordingly, in a colordisplay device having color elements of R (red), G (green) and B (blue),the minimum unit of an image is composed of three pixels of an R pixel,a G pixel, and a B pixel. Note that the color elements are not limitedto three colors, and color elements of more than three colors may beused and/or a color other than RGB may be used. For example, it ispossible to add white so that RGBW (W means white) are used.Alternatively, RGB added with one or more colors of yellow, cyan,magenta, emerald green, vermilion, and the like can be used. Further, acolor similar to at least one of R, G, and B can be added to RGB. Forexample, R, G, B1, and B2 may be employed. Although both B1 and B2 areblue, they have slightly different frequencies. Similarly, R1, R2, G,and B can be used. By using such color elements, display which is closerto a real object can be performed, and power consumption can be reduced.As another example, when brightness of one color element is controlledby a plurality of regions, one region can correspond to one pixel. Forexample, when area ratio grayscale display is performed or when asubpixel is included, a plurality of regions which control brightnessare provided in one color element and a gray level is expressed with allof the regions, and one region which controls brightness can correspondto one pixel. In that case, one color element is formed of a pluralityof pixels. Alternatively, even when a plurality of regions which controlbrightness are provided in one color element, one color elementincluding these regions may be collectively considered as one pixel. Inthat case, one color element is formed of one pixel. In addition, whenbrightness of one color element is controlled by a plurality of regions,the size of regions which contribute to display sometimes vary dependingon pixels. Alternatively, in a plurality of regions which controlbrightness in one color element, signals supplied to respective regionsmay slightly vary to widen a viewing angle. That is, potentials of pixelelectrodes included in the plurality of regions in one color element canbe different from each other. Accordingly, voltages applied to liquidcrystal molecules vary depending on the pixel electrodes. Thus, theviewing angle can be widened.

Note that when it is explicitly described as one pixel (for threecolors), it corresponds to the case where three pixels of R, G, and Bare considered as one pixel. Meanwhile, when it is explicitly describedas one pixel (for one color), it corresponds to the case where aplurality of regions provided in one color element are collectivelyconsidered as one pixel.

Note that pixels are provided (arranged) in matrix in some cases. Here,the description that pixels are provided (arranged) in matrix includesthe case where the pixels are arranged in a straight line or a jaggedline in the longitudinal direction or the lateral direction. Therefore,when full color display with three color elements (e.g., RGB) isperformed, the following cases are included therein: the case where thepixels are arranged in stripes, the case where dots of the three colorelements are arranged in a delta pattern, and the case where dots of thethree color elements are provided in Bayer arrangement. Further, thesizes of display regions may be different between respective dots ofcolor elements. Thus, power consumption can be reduced or the life of adisplay element can be prolonged.

An active matrix method in which an active element is included in apixel or a passive matrix method in which an active element is notincluded in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), a variety of active elements (non-linear elements) such as ametal-insulator-metal (MIM) and a thin film diode (TFD) can be used inaddition to a transistor. Since such an element needs a smaller numberof manufacturing steps, manufacturing costs can be reduced or the yieldcan be increased. Further, since the size of such an element is small,the aperture ratio can be increased, so that power consumption can bereduced and higher luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the manufacturing steps are fewer, so that manufacturing costs canbe reduced or the yield can be increased. Further, since an activeelement (a non-linear element) is not used, the aperture ratio can beincreased, so that power consumption can be reduced and higher luminancecan be achieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor includes a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor change depending on thestructure, operating condition, and the like of the transistor, it isdifficult to define which is a source or a drain. Thus, a region whichserves as a source or a drain is not referred to as a source or a drainin some cases. In such a case, one of the source and the drain may bereferred to as a first terminal and the other of the source and thedrain may be referred to as a second terminal, for example.Alternatively, one of the source and the drain may be referred to as afirst electrode and the other thereof may be referred to as a secondelectrode. Further alternatively, one of the source and the drain may bereferred to as a first region and the other thereof may be referred toas a second region.

Note that a transistor may be an element having at least three terminalsof a base, an emitter, and a collector. In this case also, the emitterand the collector may be referred to as a first terminal and a secondterminal, for example.

A gate corresponds to all or some of a gate electrode and a gate wiring(also called a gate line, a gate signal line, a scan line, a scan signalline, or the like). A gate electrode corresponds to part of a conductivefilm that overlaps with a semiconductor which forms a channel regionwith a gate insulating film therebetween. Note that in some cases, partof the gate electrode overlaps with an LDD (lightly doped drain) regionor a source region (or a drain region) with the gate insulating filmtherebetween. A gate wiring corresponds to a wiring for connecting gateelectrodes of transistors to each other, a wiring for connecting gateelectrodes in pixels to each other, or a wiring for connecting a gateelectrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, when a channel regionoverlaps with part of an extended gate wiring, the overlapped portion(region, conductive film, wiring, or the like) functions as both a gatewiring and a gate electrode. Accordingly, such a portion (a region, aconductive film, a wiring, or the like) may be called either a gateelectrode or a gate wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed using the same material as a gate electrode, forms thesame island as the gate electrode, and is connected to the gateelectrode may be referred to as a gate electrode. Similarly, a portion(a region, a conductive film, a wiring, or the like) which is formedusing the same material as a gate wiring, forms the same island as thegate wiring, and is connected to the gate wiring may be referred to as agate wiring. In a strict sense, such a portion (a region, a conductivefilm, a wiring, or the like) does not overlap with a channel region ordoes not have a function of connecting the gate electrode to anothergate electrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed using the samematerial as a gate electrode or a gate wiring, forms the same island asthe gate electrode or the gate wiring, and is connected to the gateelectrode or the gate wiring because of specifications or the like inmanufacturing. Thus, such a portion (a region, a conductive film, awiring, or the like) may be referred to as either a gate electrode or agate wiring.

Note that in a multi-gate transistor, for example, a gate electrode isoften connected to another gate electrode by using a conductive filmwhich is formed using the same material as the gate electrode. Sincesuch a portion (a region, a conductive film, a wiring, or the like) is aportion (a region, a conductive film, a wiring, or the like) forconnecting the gate electrode to another gate electrode, the portion maybe referred to as a gate wiring, or the portion may be referred to as agate electrode because a multi-gate transistor can be considered as onetransistor. That is, a portion (a region, a conductive film, a wiring,or the like) which is formed using the same material as a gate electrodeor a gate wiring, forms the same island as the gate electrode or thegate wiring, and is connected to the gate electrode or the gate wiringmay be referred to as either a gate electrode or a gate wiring. Further,for example, part of a conductive film which connects the gate electrodeand the gate wiring and is formed using a material which is differentfrom that of the gate electrode or the gate wiring may be referred to aseither a gate electrode or a gate wiring.

Note that a gate terminal corresponds to part of a portion (a region, aconductive film, a wiring, or the like) of a gate electrode or part of aportion (a region, a conductive film, a wiring, or the like) which iselectrically connected to the gate electrode.

In the case where a wiring is referred to as a gate wiring, a gate line,a gate signal line, a scan line, a scan signal line, or the like, a gateof a transistor is not connected to the wiring in some cases. In thiscase, the gate wiring, the gate line, the gate signal line, the scanline, or the scan signal line sometimes corresponds to a wiring formedin the same layer as the gate of the transistor, a wiring formed usingthe same material as the gate of the transistor, or a wiring formed atthe same time as the gate of the transistor. Examples are a wiring for astorage capacitor, a power supply line, and a reference potential supplyline.

A source corresponds to all or some of a source region, a sourceelectrode, and a source wiring (also referred to as a source line, asource signal line, a data line, a data signal line, or the like). Asource region corresponds to a semiconductor region containing a largeamount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Therefore, a region containinga small amount of p-type impurities or n-type impurities, that is, anLDD (lightly doped drain) region is not included in the source region. Asource electrode is part of a conductive layer which is formed using amaterial different from that of a source region and is electricallyconnected to the source region. Note that a source electrode and asource region are collectively referred to as a source electrode in somecases. A source wiring corresponds to a wiring for connecting sourceelectrodes of transistors to each other, a wiring for connecting sourceelectrodes of pixels to each other, or a wiring for connecting a sourceelectrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which serves as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe referred to as either a source electrode or a source wiring. That is,there is a region in which a source electrode and a source wiring cannotbe clearly distinguished from each other. For example, in the case wherea source region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive film, wiring, or the like) servesas both a source wiring and a source electrode. Thus, such a portion (aregion, a conductive film, a wiring, or the like) may be referred to aseither a source electrode or a source wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed using the same material as a source electrode, forms thesame island as the source electrode, and is connected to the sourceelectrode; or a portion (a region, a conductive film, a wiring, or thelike) which connects a source electrode and another source electrode maybe referred to as a source electrode. Further, a portion which overlapswith a source region may be referred to as a source electrode.Similarly, a region which is formed using the same material as a sourcewiring, forms the same island as the source wiring, and is connected tothe source wiring may be referred to as a source wiring. In a strictsense, such a portion (a region, a conductive film, a wiring, or thelike) does not have a function of connecting the source electrode toanother source electrode in some cases. However, there is a portion (aregion, a conductive film, a wiring, or the like) which is formed usingthe same material as a source electrode or a source wiring, forms thesame island as the source electrode or the source wiring, and isconnected to the source electrode or the source wiring because ofspecifications or the like in manufacturing. Thus, such a portion (aregion, a conductive film, a wiring, or the like) may be referred to aseither a source electrode or a source wiring.

For example, part of a conductive film which connects the sourceelectrode and the source wiring and is formed using a material which isdifferent from that of the source electrode or the source wiring may bereferred to as either a source electrode or a source wiring.

A source terminal corresponds to part of a source region, part of asource electrode, or part of a portion (a region, a conductive film, awiring, or the like) which is electrically connected to the sourceelectrode.

In the case where a wiring is referred to as a source wiring, a sourceline, a source signal line, a data line, a data signal line, or thelike, a source (a drain) of a transistor is not connected to the wiringin some cases. In that case, the source wiring, the source line, thesource signal line, the data line, or the data signal line sometimescorresponds to a wiring formed in the same layer as the source (thedrain) of the transistor, a wiring formed using the same material as thesource (the drain) of the transistor, or a wiring formed at the sametime as the source (the drain) of the transistor. Examples are a wiringfor a storage capacitor, a power supply line, and a reference potentialsupply line.

Note also that the same can be said for a drain.

A semiconductor device corresponds to a device having a circuitincluding a semiconductor element (e.g., a transistor, a diode, or athyristor). The semiconductor device may be general devices that canfunction by utilizing semiconductor characteristics. Alternatively,devices including a semiconductor material are also referred to assemiconductor devices.

A display device corresponds to a device including a display element. Adisplay device may include a plurality of pixels including a displayelement. Moreover, a display device may include a peripheral drivercircuit for driving a plurality of pixels. The peripheral driver circuitfor driving a plurality of pixels may be formed over the same substrateas the plurality of pixels. A display device may also include aperipheral driver circuit provided over a substrate by wire bonding orbump bonding, that is, an IC chip connected by chip on glass (COG), TAB,or the like. Further, a display device may include a flexible printedcircuit (FPC) to which an IC chip, a resistor, a capacitor, an inductor,a transistor, or the like is attached. A display device may include aprinted wiring board (PWB) which is connected through a flexible printedcircuit (FPC) and to which an IC chip, a resistor, a capacitor, aninductor, a transistor, or the like is attached. A display device mayalso include an optical sheet such as a polarizing plate or aretardation plate. A display device may also include a lighting device,a housing, an audio input/output device, a light sensor, or the like.

A lighting device may include a backlight unit, a light guide plate, aprism sheet, a diffusion sheet, a reflective sheet, a light source(e.g., an LED or a cold cathode fluorescent lamp), a cooling device(e.g., a water cooling device or an air cooling device), or the like.

A light-emitting device corresponds to a device including alight-emitting element or the like. A light-emitting device including alight-emitting element as a display element is a specific example ofdisplay devices.

A reflective device corresponds to a device including a light-reflectingelement, a light diffraction element, a light-reflecting electrode, orthe like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection type liquidcrystal display, a transmissive liquid crystal display, a reflectiveliquid crystal display, a transflective liquid crystal display, and thelike in their categories.

A driving device corresponds to a device including a semiconductorelement, an electric circuit, an electronic circuit, or the like.Examples of the driving device are a transistor which controls input ofa signal from a source signal line to a pixel (also referred to as aselection transistor, a switching transistor, or the like), a transistorwhich supplies voltage or current to a pixel electrode, and a transistorwhich supplies voltage or current to a light-emitting element. Moreover,examples of the driving device are a circuit which supplies a signal toa gate signal line (also referred to as a gate driver, a gate linedriver circuit, or the like) and a circuit which supplies a signal to asource signal line (also referred to as a source driver, a source linedriver circuit, or the like).

Note that categories of a display device, a semiconductor device, alighting device, a cooling device, a light-emitting device, a reflectivedevice, a driving device, and the like overlap with each other in somecases. For example, a display device includes a semiconductor device anda light-emitting device in some cases. Alternatively, a semiconductordevice includes a display device and a driving device in some cases.

Note that when it is explicitly described that B is formed on or over A,it does not necessarily mean that B is formed in direct contact with A.The description includes the case where A and B are not in directcontact with each other, that is, the case where another object isplaced between A and B. Here, each of A and B is an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer).

Accordingly, for example, when it is explicitly described that a layer Bis formed on (or over) a layer A, it includes both the case where thelayer B is formed in direct contact with the layer A; and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A, and the layer B is formed in direct contact with thelayer C or the layer D. Note that another layer (e.g., the layer C orthe layer D) may be a single layer or a plurality of layers.

Similarly, when it is explicitly described that B is formed above A, itdoes not necessarily mean that B is formed in direct contact with A, andanother object may be placed between A and B. Accordingly, the casewhere a layer B is formed above a layer A includes the case where thelayer B is formed in direct contact with the layer A and the case whereanother layer (e.g., a layer C and a layer D) is formed in directcontact with the layer A and the layer B is formed in direct contactwith the layer C or the layer D. Note that another layer (e.g., thelayer C or the layer D) may be a single layer or a plurality of layers.

Note that when it is explicitly described that B is formed over, on, orabove A, it includes the case where B is formed obliquely over/above A.

Note that the same can be said when it is explicitly described that B isformed below or under A.

Explicit singular forms preferably mean singular forms. However,embodiments of the present invention are not limited thereto, and suchsingular forms can include plural forms. Similarly, explicit pluralforms preferably mean plural forms. However, embodiments of the presentinvention are not limited thereto, and such plural forms can includesingular forms.

Note that the size, the thickness of layers, and regions in diagrams aresometimes exaggerated for simplicity. Therefore, embodiments of thepresent invention are not limited to such scales.

Note that a diagram schematically illustrates an ideal example, andembodiments of the present invention are not limited to the shape or thevalue illustrated in the diagram. For example, the following can beincluded: variation in shape due to a manufacturing technique ordimensional deviation; or variation in signal, voltage, or current dueto noise or difference in timing.

Technical terms are used in order to describe a specific embodiment orthe like in many cases, and there are no limitations on terms.

Terms which are not defined (including terms used for science andtechnology, such as technical terms and academic parlance) can be usedas the terms which have a meaning equivalent to a general meaning thatan ordinary person skilled in the art understands. It is preferable thatthe term defined by dictionaries or the like be construed as aconsistent meaning with the background of related art.

The terms such as first, second, and third are used for distinguishingvarious elements, members, regions, layers, and areas from others.Therefore, the terms such as first, second, and third do not limit thenumber of elements, members, regions, layers, areas, or the like.Further, for example, “first” can be replaced with “second”, “third”, orthe like.

Terms for describing spatial arrangement, such as “over”, “above”,“under”, “below”, “laterally”, “right”, “left”, “obliquely”, “back”, and“front”, are often used for briefly showing, with reference to adiagram, a relation between an element and another element or betweensome characteristics and other characteristics. Note that embodiments ofthe present invention are not limited thereto, and such terms fordescribing spatial arrangement can indicate not only the directionillustrated in a diagram but also another direction. For example, whenit is explicitly described that “B is over A”, it does not necessarilymean that B is placed over A, and can include the case where B is placedunder A because a device in a diagram can be inverted or rotated by180°. Accordingly, “over” can refer to the direction described by“under” in addition to the direction described by “over”. Note thatembodiments of the present invention are not limited thereto, and “over”can refer to other directions described by “laterally”, “right”, “left”,“obliquely”, “back”, and “front” in addition to the directions describedby “over” and “under” because a device in a diagram can be rotated in avariety of directions.

In one embodiment of the invention disclosed in this specification andthe like, a light-transmitting material is used for at least part of atransistor and part of a storage capacitor. Accordingly, light can passthrough a region where the transistor and the storage capacitor areprovided, so that the aperture ratio can be increased. Further, when awiring for connecting a transistor and another element (e.g., anothertransistor) or a wiring for connecting a capacitor and another element(e.g., another capacitor) is formed using a material with lowresistivity (with high conductivity), distortion of signal waveforms canbe reduced and a voltage drop due to wiring resistance can be reduced.Thus, power consumption of a semiconductor device can be reduced.Moreover, the size of the semiconductor device (the size of a screen)can be easily increased.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a plan view and FIG. 1B is a cross-sectional view of asemiconductor device;

FIGS. 2A1 to 2D1 and 2A2 to 2D2 are cross-sectional views illustrating amethod for manufacturing a semiconductor device;

FIGS. 3A1 to 3D1 and 3A2 to 3D2 are cross-sectional views illustrating amethod for manufacturing a semiconductor device;

FIGS. 4A1 to 4D1 and 4A2 to 4D2 are cross-sectional views illustrating amethod for manufacturing a semiconductor device;

FIGS. 5A1 to 5C1 and 5A2 to 5C2 are cross-sectional views illustrating amethod for manufacturing a semiconductor device;

FIG. 6A is a plan view and FIG. 6B is a cross-sectional view of asemiconductor device;

FIG. 7A is a plan view and FIG. 7B is a cross-sectional view of asemiconductor device;

FIG. 8A is a plan view and FIG. 8B is a cross-sectional view of asemiconductor device;

FIG. 9A is a plan view and FIG. 9B is a cross-sectional view of asemiconductor device;

FIG. 10A is a plan view and FIG. 10B is a cross-sectional view of asemiconductor device;

FIG. 11A is a plan view and FIG. 11B is a cross-sectional view of asemiconductor device;

FIG. 12A is a plan view and FIG. 12B is a cross-sectional view of asemiconductor device;

FIGS. 13A1 to 13D1 and 13A2 to 13D2 are cross-sectional viewsillustrating a method for manufacturing a semiconductor device;

FIGS. 14A1 to 14C1 and 14A2 to 14C2 are cross-sectional viewsillustrating a method for manufacturing a semiconductor device;

FIGS. 15A1 to 15C1 and 15A2 to 15C2 are cross-sectional viewsillustrating a method for manufacturing a semiconductor device;

FIGS. 16A1 to 16C1 and 16A2 to 16C2 are cross-sectional viewsillustrating a method for manufacturing a semiconductor device;

FIGS. 17A1, 17A2, 17B1, and 17B2 each illustrate a structure of amulti-tone mask;

FIG. 18A is a plan view and FIG. 18B is a cross-sectional view of asemiconductor device;

FIG. 19A is a plan view and FIG. 19B is a cross-sectional view of asemiconductor device;

FIG. 20A is a plan view and FIG. 20B is a cross-sectional view of asemiconductor device;

FIG. 21A is a plan view and FIG. 21B is a cross-sectional view of asemiconductor device;

FIGS. 22A1 and 22A2 are plan views and FIG. 22B is a cross-sectionalview of a semiconductor device;

FIG. 23 illustrates a semiconductor device;

FIG. 24 is a cross-sectional view illustrating a semiconductor device;

FIGS. 25A to 25C are cross-sectional views each illustrating asemiconductor device;

FIG. 26A is a plan view and FIG. 26B is a cross-sectional view of asemiconductor device;

FIGS. 27A and 27B each illustrate a semiconductor device;

FIGS. 28A and 28B are cross-sectional views each illustrating asemiconductor device;

FIGS. 29A and 29B each illustrate an example of application ofelectronic paper;

FIG. 30 is an external view illustrating an example of an electronicbook reader;

FIGS. 31A and 31B are external views of an example of a televisiondevice and a digital photo frame, respectively;

FIGS. 32A and 32B are external views each illustrating an example of anamusement machine;

FIGS. 33A and 33B are external views each illustrating an example of amobile phone;

FIGS. 34A to 34C are cross-sectional views illustrating a method formanufacturing a semiconductor device;

FIGS. 35A and 35B are cross-sectional views each illustrating asemiconductor device;

FIGS. 36A to 36C are cross-sectional views illustrating a method formanufacturing a semiconductor device;

FIG. 37A is a plan view and FIG. 37B is a cross-sectional view of asemiconductor device;

FIG. 38A is a plan view and FIG. 38B is a cross-sectional view of asemiconductor device;

FIGS. 39A and 39B each illustrate a semiconductor device;

FIGS. 40A to 40G each illustrate a semiconductor device;

FIGS. 41A to 41H each illustrate a semiconductor device;

FIGS. 42A to 42F each illustrate a semiconductor device; and

FIGS. 43A to 43C each illustrate a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings. Note that the present inventionis not limited to the description of the embodiments, and it is apparentto those skilled in the art that modes and details can be modified invarious ways without departing from the spirit of the inventiondisclosed in this specification and the like. In addition, thestructures in different embodiments can be implemented in combination asappropriate. Note that the same portions or portions having similarfunctions are denoted by the same reference numerals in the structure ofthe invention described below, and the description thereof is repeated.

Note that what is described (or part thereof) in one embodiment can beapplied to, combined with, or exchanged with another content in the sameembodiment and/or what is described (or part thereof) in anotherembodiment or other embodiments.

Note that in each embodiment, what is described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with texts disclosed in this specification.

In addition, by combining a diagram (or part thereof) described in oneembodiment with another part of the diagram, a different diagram (orpart thereof) described in the same embodiment, and/or a diagram (orpart thereof) described in one or a plurality of different embodiments,much more diagrams can be formed.

Note that in a diagram or a text described in one embodiment, it ispossible to take out part of the diagram or the text and constitute anembodiment of the invention. Accordingly, in the case where a diagram ora text related to a certain portion is described, the context taken outfrom part of the diagram or the text is also disclosed as one embodimentof the invention and can constitute one embodiment of the invention.Therefore, for example, in a diagram (e.g., a cross-sectional view, aplan view, a circuit diagram, a block diagram, a flow chart, a processdiagram, a perspective view, a cubic diagram, a layout diagram, a timingchart, a structure diagram, a schematic view, a graph, a list, a raydiagram, a vector diagram, a phase diagram, a waveform chart, aphotograph, or a chemical formula) or a text in which one or more activeelements (e.g., transistors or diodes), wirings, passive elements (e.g.,capacitors or resistors), conductive layers, insulating layers,semiconductor layers, organic materials, inorganic materials,components, substrates, modules, devices, solids, liquids, gases,operating methods, manufacturing methods, or the like are described, itis possible to take out part of the diagram or the text and constituteone embodiment of the invention.

Embodiment 1

In this embodiment, a semiconductor device and a method formanufacturing the semiconductor device will be described with referenceto FIGS. 1A and 1B, FIGS. 2A1 to 2D1 and 2A2 to 2D2, FIGS. 3A1 to 3D1and 3A2 to 3D2, FIGS. 4A1 to 4D1 and 4A2 to 4D2, FIGS. 5A1 to 5C1 and5A2 to 5C2, FIGS. 6A and 6B, FIGS. 7A and 7B, FIGS. 8A and 8B, FIGS. 9Aand 9B, FIGS. 10A and 10B, and FIGS. 11A and 11B.

FIGS. 1A and 1B illustrate an example of a structure of a semiconductordevice according to this embodiment. A liquid crystal display device isspecifically described as a semiconductor device in this embodiment.However, the disclosed invention is not limited to a liquid crystaldisplay device. The disclosed invention can be applied to anelectroluminescent display device (an EL display device), a displaydevice using an electrophoretic element (i.e., a so-called electronicpaper), and the like and also applied to a semiconductor device otherthan a display device. FIG. 1A is a plan view, and FIG. 1B is across-sectional view along A-B in FIG. 1A.

The semiconductor device illustrated in FIG. 1A includes a pixel portionincluding a conductive layer 112 functioning as a source wiring; aconductive layer 132 a and a conductive layer 132 b which intersect theconductive layer 112 and function as a gate wiring and a capacitorwiring, respectively; a transistor 150 near the intersection of theconductive layer 132 a and the conductive layer 112; and a storagecapacitor 152 electrically connected to the conductive layer 132 b (seeFIGS. 1A and 1B). Note that in this specification and the like, thepixel portion refers to a region surrounded by a conductive layerfunctioning as a gate wiring and a conductive layer functioning as asource wiring. In FIG. 1A, the conductive layer 112 and the conductivelayers 132 a and 132 b intersect at 90°; however, the disclosedinvention is not limited to this structure. That is, the conductivelayer 112 and the conductive layers 132 a and 132 b may intersect at anangle other than 90°.

The transistor 150 is a so-called top-gate transistor including aconductive layer 106 a functioning as a source electrode, a conductivelayer 106 b functioning as a drain electrode, a semiconductor layer 118a, a gate insulating layer 120, and a conductive layer 126 a functioningas a gate electrode (see FIGS. 1A and 1B). The storage capacitor 152includes the conductive layer 106 b, the gate insulating layer 120, aconductive layer 126 b, and a conductive layer 140. Specifically, thecapacitance is formed between the conductive layer 106 b and theconductive layer 126 b, and between the conductive layer 126 b and theconductive layer 140. Note that since functions of the source electrodeand the drain electrode of the transistor are sometimes replaced witheach other depending on the direction in which carriers flow, the terms“source electrode” and “drain electrode” are used only for convenience.In other words, a function of each conductive layer should not beconstrued as being limited to the above terms.

Here, the conductive layer 106 a, the conductive layer 106 b, thesemiconductor layer 118 a, and the conductive layer 126 a which areincluded in the transistor 150, and the conductive layer 126 b includedin the storage capacitor 152 are formed using a light-transmittingmaterial. Accordingly, the aperture ratio of a pixel can be increased.

The conductive layer 112 electrically connected to the conductive layer106 a and the conductive layer 132 a electrically connected to theconductive layer 126 a are formed using a low resistance material.Accordingly, wiring resistance can be reduced, and power consumption canbe reduced. Moreover, the conductive layer 112 and the conductive layer132 a are formed using a material with a light-shielding property. Thus,a portion between pixels can be shielded from light.

Note that in the above description, the term “light-transmitting” meansthat the light transmittance in the visible range (approximately 400 nmto 800 nm) is higher than at least that of the conductive layer 112 andthe conductive layer 132 a.

Next, an example of a method for manufacturing the semiconductor devicewill be described.

First, a conductive layer 102 is formed over a substrate 100 having aninsulating surface (see FIGS. 2A1 and 2A2).

As the substrate 100 having the insulating surface, a glass substratewith a visible light-transmitting property, which is used for a liquidcrystal display device or the like, can be used, for example. The glasssubstrate is preferably a non-alkali glass substrate. For the non-alkaliglass substrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. Moreover, as the substrate 100 having the insulating surface,an insulating substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate; a semiconductorsubstrate which is formed using a semiconductor material such as siliconand has a surface covered with an insulating material; a conductivesubstrate which is formed using a conductive material such as metal orstainless steel and has a surface covered with an insulating material;or the like can be used. A flexible synthetic resin typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES) may be used.

Although not shown, a base film is preferably provided over thesubstrate 100 having the insulating surface. The base film has afunction of preventing diffusion of alkali metal (e.g., Li, Cs, or Na),alkaline earth metal (e.g., Ca or Mg), or other impurities from thesubstrate 100. In other words, by providing the base film, the object ofimproving the reliability of the semiconductor device can be achieved.The base film can be formed using one or a plurality of insulatinglayers such as a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, or an aluminumnitride oxide film. For example, the base film preferably has astructure in which a silicon nitride film and a silicon oxide film aresequentially stacked from the substrate side. This is because thesilicon nitride film is highly effective in blocking impurities.Meanwhile, since defects might occur in the case where the siliconnitride film is in contact with a semiconductor, a silicon oxide film ispreferably formed as a film in contact with the semiconductor.

Note that in this specification and the like, an oxynitride refers to asubstance containing a larger amount (number of atoms) of oxygen thannitrogen. For example, a silicon oxynitride contains oxygen, nitrogen,silicon, and hydrogen at concentration ranging from 50 at.% to 70 at.%,0.5 at.% to 15 at.%, 25 at.% to 35 at.%, and 0.1 at.% to 10 at.%,respectively. Further, a nitride oxide refers to a substance containinga larger amount (number of atoms) of nitrogen than oxygen. For example,a nitride oxide contains oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 5 at.% to 30 at.%, 20 at.% to 55 at.%, 25at.% to 35 at.%, and 10 at.% to 25 at.%, respectively. Note that theabove ranges are obtained by using Rutherford backscatteringspectrometry (RBS) or hydrogen forward scattering (HFS). Moreover, thetotal of the content ratio of the constituent elements does not exceed100 at.%.

The conductive layer 102 is preferably formed using a material with alight-transmitting property (a visible light-transmitting property),such as indium tin oxide (ITO), indium tin oxide containing siliconoxide (ITSO), organoindium, organotin, zinc oxide (ZnO), or titaniumnitride. Alternatively, indium zinc oxide (IZO) containing zinc oxide, amaterial obtained by adding gallium (Ga) to zinc oxide, tin oxide(SnO₂), indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, or the like may be used. Theconductive layer 102 may have a signal-layer structure or a layeredstructure. When a layered structure is employed, the conductive layer102 is preferably formed so as to have a sufficiently high lighttransmittance. Note that a sputtering method is preferably used as amethod for forming the conductive layer 102; however, this embodiment isnot limited thereto.

Next, a resist mask 104 a and a resist mask 104 b are formed over theconductive layer 102, and the conductive layer 102 is selectively etchedusing the resist masks 104 a and 104 b, so that the conductive layer 106a and the conductive layer 106 b are formed (see FIGS. 2B1 and 2B2). Asthe etching, either dry etching or wet etching may be used. After theetching, the resist masks 104 a and 104 b are removed. In order toimprove the coverage of the conductive layers 106 a and 106 b with aninsulating layer and the like which are formed later and preventdisconnection, it is preferable to form the conductive layers 106 a and106 b with their end portions tapered. By forming the conductive layersto be tapered in such a manner, the object of increasing the yield ofthe semiconductor device can be achieved.

The conductive layer 106 a functions as the source electrode of thetransistor. The conductive layer 106 b functions as the drain electrodeof the transistor and an electrode (a capacitor electrode) of thestorage capacitor. Note that a function of each conductive layer shouldnot be construed as being limited to the term “source electrode” or“drain electrode”.

Next, a conductive layer 108 is formed so as to cover the conductivelayers 106 a and 106 b (see FIGS. 2C1 and 2C2). Note that the conductivelayer 108 is formed so as to cover the conductive layers 106 a and 106 bhere; however, the disclosed invention is not limited thereto.

The conductive layer 108 can be formed to have a single-layer structureor a layered structure using a metal element such as aluminum (Al),tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel(Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese(Mn), neodymium (Nd), niobium (Nb), chromium (Cr), or cerium (Ce); analloy material containing such metal materials as main components; or anitride containing such a metal material. For example, the conductivelayer 108 is preferably formed using a low resistance material such asaluminum.

In the case where the conductive layer 108 is formed over the conductivelayer 106 a, these conductive layers might react with each other. Forexample, when ITO is used for the conductive layer 106 a and aluminum isused for the conductive layer 108, chemical reaction might be caused. Inorder to prevent such reaction, the conductive layer 108 may have alayered structure of a high melting point material and a low resistancematerial. Specifically, for the layered structure of the conductivelayer 108, it is preferable that a region which is in contact with theconductive layer 106 a be formed using a high melting point material anda region which is not in contact with the conductive layer 106 a beformed using a low resistance material, for example.

Examples of the high melting point material are molybdenum, titanium,tungsten, tantalum, and chromium. Examples of the low resistancematerial are aluminum, copper, and silver.

It is needless to say that the conductive layer 108 may have a layeredstructure of three or more layers. In that case, a layered structure inwhich molybdenum, aluminum, molybdenum are used for a first layer, asecond layer, and a third layer, respectively; or a layered structure inwhich molybdenum, aluminum containing a small amount of neodymium, andmolybdenum are used for a first layer, a second layer, and a thirdlayer, respectively can be used. When the conductive layer 108 has sucha layered structure, generation of hillocks can be prevented.Accordingly, the object of improving the reliability of thesemiconductor device can be achieved.

Then, a resist mask 110 is formed over the conductive layer 108, and theconductive layer 108 is selectively etched using the resist mask 110, sothat the conductive layer 112 is formed (see FIGS. 2D1 and 2D2). Notethat the conductive layer 112 functions as the source wiring. Moreover,since the conductive layer 112 is formed using a material with alight-shielding property, the conductive layer 112 has a function ofshielding light. The resist mask 110 is removed after the conductivelayer 112 is formed.

Note that the steps for forming the conductive layer 112 after theconductive layers 106 a and 106 b are formed are described in thisembodiment; however, the disclosed invention should not be construed asbeing limited thereto. For example, the order of formation of theconductive layers 106 a and 106 b and formation of the conductive layer112 may be changed. That is, the conductive layer 106 a functioning asthe source electrode and the conductive layer 106 b can be formed afterthe conductive layer 112 functioning as the source wiring is formed (seeFIGS. 6A and 6B). Note that although the order of formation of theconductive layers 126 a and 126 b and formation of the conductive layers132 a and 132 b is not changed in FIGS. 6A and 6B, the order may bechanged.

When the conductive layer 112 is formed by etching of the conductivelayer 108, a conductive layer 113 may be formed in a region where acontact hole is formed later (see FIGS. 7A and 7B). By employing such astructure, the region where a contact hole is formed can be shieldedfrom light. Accordingly, display defects due to unevenness of a surfaceof an electrode (a pixel electrode) in a contact region can be reduced,whereby the effect of increasing the contrast and reducing light leakagecan be obtained. That is, the object of improving displaycharacteristics can be achieved. Note that although this structure isparticularly effective in a liquid crystal display device, it isneedless to say that this structure can be applied to othersemiconductor devices. In that case, the conductive layer 113 may beformed as appropriate in a region where light needs to be blocked.

Next, a semiconductor layer 114 is formed so as to cover at least theconductive layers 106 a and 106 b (see FIGS. 3A1 and 3A2). In thisembodiment, the semiconductor layer 114 is formed over the substrate 100so as to cover the conductive layers 106 a and 106 b and the conductivelayer 112.

The semiconductor layer 114 can be formed using an In—Ga—Zn—O-basedoxide semiconductor material or various kinds of oxide semiconductormaterials such as an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, aSn—Al—Zn—O-based oxide semiconductor, an In—Zn—O-based oxidesemiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-basedoxide semiconductor, and a Zn—O-based oxide semiconductor.Alternatively, other materials can be used. For example, thesemiconductor layer 114 formed using an In—Ga—Zn—O-based oxidesemiconductor material can be formed by a sputtering method using anoxide semiconductor target containing In, Ga, and Zn(In₂O₃:Ga₂O₃:ZnO=1:1:1). The condition of sputtering can be set asfollows, for example: the distance between the substrate 100 and thetarget is 30 mm to 500 mm, the pressure is 0.1 Pa to 2.0 Pa, the directcurrent (DC) power supply is 0.25 kW to 5.0 kW (when a target of 8inches in diameter is used), and the atmosphere is an argon atmosphere,an oxygen atmosphere, or a mixed atmosphere of argon and oxygen. Notethat a ZnO-based non-single-crystal film may be used as thesemiconductor layer 114. The semiconductor layer 114 may have athickness of approximately 5 nm to 200 nm.

As the above sputtering method, an RF sputtering method using a highfrequency power supply for a power supply for sputtering, a DCsputtering method, a pulsed DC sputtering method in which a DC bias isapplied in a pulse manner, or the like can be employed. Note that apulsed direct current (DC) power supply is preferably used because dustcan be reduced and thickness distribution becomes uniform. In that case,the object of increasing the yield and the reliability of thesemiconductor device can be achieved.

Alternatively, a multi-source sputtering apparatus in which a pluralityof targets of different materials can be set may be used. In themulti-source sputtering apparatus, a plurality of films can be formed inthe same chamber, or one film can be formed by sputtering plural kindsof materials in one chamber at the same time. Moreover, a method ofusing a magnetron sputtering apparatus in which a magnetic fieldgenerating system is provided inside a chamber (a magnetron sputteringmethod), an ECR sputtering method in which plasma generated by usingmicrowaves is used, or the like may be employed. Furthermore, a reactivesputtering method in which a target substance and a sputtering gascomponent chemically react with each other during deposition to form acompound, a bias sputtering method in which voltage is also applied to asubstrate during deposition, or the like may be used.

Before the semiconductor layer 114 is formed, plasma treatment may beperformed on a surface over which the semiconductor layer 114 is to beformed (e.g., surfaces of the conductive layers 106 a and 106 b and, inthe case where the base film is formed, a surface of the base film).With the plasma treatment, dust attached to the surface over which thesemiconductor layer 114 is to be formed can be removed. Moreover, byforming the semiconductor layer 114 without exposure to air after theplasma treatment is performed, the conductive layers 106 a and 106 b canbe electrically connected to the semiconductor layer 114 in a favorablemanner. In other words, the object of increasing the yield and thereliability of the semiconductor device can be achieved.

In this embodiment, the case where an oxide semiconductor material isused for the semiconductor layer 114 is described; however, oneembodiment of the disclosed invention is not limited thereto. If asemiconductor material other than an oxide semiconductor material, acompound semiconductor material, or the like is used, the semiconductorlayer can have light transmittance in some cases when its thickness ismade smaller. Accordingly, another semiconductor material may be usedinstead of an oxide semiconductor material. Examples of anothersemiconductor material are various kinds of inorganic semiconductormaterials such as silicon, gallium, and gallium arsenide; an organicsemiconductor material such as a carbon nanotube; and a material inwhich such materials are mixed. Such a material with a variety of modessuch as single crystallinity, polycrystallinity, microcrystallinity(including microcrystal state and nanocrystal state), and amorphousstate can be used for the semiconductor layer 114.

Next, a resist mask 116 a and a resist mask 116 b are formed over thesemiconductor layer 114, and the semiconductor layer 114 is selectivelyetched using the resist masks 116 a and 116 b, so that the semiconductorlayer 118 a and the semiconductor layer 118 b are formed (see FIGS. 3B1and 3B2). The semiconductor layers 118 a and 118 b are formed in islandshapes. Here, the semiconductor layer 118 a serves as an active layer ofthe transistor. The semiconductor layer 118 b serves to reduce theparasitic resistance between wirings. Note that the case where thesemiconductor layer 118 b is formed is described in this embodiment;however, the semiconductor layer 118 b is not necessarily formed.

The resist mask may be formed by a spin coating method or the like. Whena droplet discharging method, a screen printing method, or the like isused, the resist mask can be selectively formed. In that case, theobject of increasing the productivity can be achieved.

Wet etching or dry etching can be used for etching of the semiconductorlayer 114. Here, an unnecessary portion of the semiconductor layer 114is removed by wet etching using a mixed solution of acetic acid, nitricacid, and phosphoric acid; thus, the semiconductor layers 118 a and 118b are formed. After the etching, the resist masks 116 a and 116 b areremoved. An etchant (an etchant solution) used for the wet etching isnot limited to the above solution as long as the semiconductor layer 114can be etched using the etchant.

In the case of performing dry etching, a gas containing chlorine or agas containing chlorine to which oxygen is added is preferably used.This is because the etching selectivity of the semiconductor layer 114to the conductive layer and the base film is likely to be high by usinga gas containing chlorine and oxygen.

As an etching apparatus used for the dry etching, an etching apparatususing a reactive ion etching method (an RIE method), or a dry etchingapparatus using a high-density plasma source such as ECR (electroncyclotron resonance) or ICP (inductively coupled plasma) can be used.Moreover, an etching apparatus with an ECCP (enhanced capacitivelycoupled plasma) mode may be used, by which electric discharge is likelyto be homogeneous in a larger area as compared to the case of an ICPetching apparatus. The ECCP mode etching apparatus can be applied evenwhen a substrate of the tenth generation or later is used.

Note that when the semiconductor layer 118 a is formed over theconductive layer 106 a functioning as the source electrode of thetransistor and the conductive layer 106 b functioning as the drainelectrode of the transistor, the semiconductor layer 118 a is easilythinned as shown in this embodiment. This is because, in the case wherethe semiconductor layer 118 a is placed over the conductive layers 106 aand 106 b, unlike in the case where the semiconductor layer 118 a isplaced below the conductive layers 106 a and 106 b, the semiconductorlayer 118 a is not removed by over-etching at the time when theconductive layer is etched. Reduction in thickness of the semiconductorlayer 118 a is realized in such a manner, whereby depletion can beeasily realized at the time of applying voltage, and the S value can bereduced. Moreover, the off-state current can be reduced. In other words,the object of realizing higher performance of the semiconductor devicecan be achieved. Note that the semiconductor layer 118 a is preferablyformed thinner than the conductive layer 112 functioning as the sourcewiring, the conductive layer 106 a functioning as the source electrode,the conductive layer 132 a functioning as the gate wiring, theconductive layer 126 a functioning as the gate electrode, and the like.

After that, heat treatment at 200° C. to 600° C., typically 300° C. to500° C., is preferably performed. Here, heat treatment is performed at350° C. for an hour in a nitrogen atmosphere. With the heat treatment,semiconductor characteristics of the semiconductor layers 118 a and 118b can be improved. Note that there is no particular limitation on thetiming of the heat treatment as long as the heat treatment is performedafter the semiconductor layers 118 a and 118 b are formed.

Note that the steps in which the conductive layer 112 is formed afterthe conductive layers 106 a and 106 b are formed, and then thesemiconductor layer 118 a is formed are described in this embodiment;however, the disclosed invention should not be construed as beinglimited thereto. For example, after the conductive layers 106 a and 106b are formed, the semiconductor layer 118 a may be formed and then theconductive layer 112 may be formed (see FIGS. 8A and 8B). This structureis effective in reducing the contact resistance with the semiconductorlayer 118 a.

Note that the conductive layers 106 a and 106 b are preferably formedthinner than the conductive layer 112. It is advantageous to make theconductive layers 106 a and 106 b thinner because, although theresistance is increased, the transmittance can be further increased. Itis needless to say that one embodiment of the disclosed invention shouldnot be construed as being limited thereto.

Next, the gate insulating layer 120 is formed so as to cover thesemiconductor layers 118 a and 118 b (see FIGS. 3C1 and 3C2).

The gate insulating layer 120 can be formed to have a single-layerstructure or a layered structure of a silicon oxide film, a siliconoxynitride film, a silicon nitride film, a silicon nitride oxide film,an aluminum oxide film, an aluminum nitride film, an aluminum oxynitridefilm, an aluminum nitride oxide film, or a tantalum oxide film. The gateinsulating layer 120 can be formed to a thickness of 50 nm to 250 nm bya sputtering method, a CVD method, or the like. Here, as the gateinsulating layer 120, a silicon oxide film is formed to a thickness of100 nm by a sputtering method. Note that the gate insulating layer 120preferably has a light-transmitting property.

Then, a conductive layer 122 is formed over the gate insulating layer120 (see FIGS. 3D1 and 3D2). The conductive layer 122 can be formedusing a material and a method which are similar to those of theconductive layer 102. Since the description of the conductive layer 102can be referred to for the details of the conductive layer 122, thedescription is not repeated. Note that the conductive layer 122preferably has a light-transmitting property.

When the conductive layer 102 and the conductive layer 122 are formedusing the same material, it becomes easy to use a material and amanufacturing device in common, which contributes to reduction in cost,increase in throughput, and the like. Needless to say, it is notessential that that the conductive layer 102 and the conductive layer122 are formed using the same material.

Next, a resist mask 124 a and a resist mask 124 b are formed over theconductive layer 122, and the conductive layer 122 is selectively etchedusing the resist masks 124 a and 124 b, so that the conductive layer 126a and the conductive layer 126 b are formed (see FIGS. 4A1 and 4A2). Asthe etching, either dry etching or wet etching may be used. After theetching, the resist masks 124 a and 124 b are removed. The conductivelayer 126 a functions as the gate electrode of the transistor. Theconductive layer 126 b functions as the electrode (the capacitorelectrode) of the storage capacitor.

Note that the area of a region where the conductive layer 106 b and theconductive layer 126 b overlap with each other can be changed asappropriate. Since the conductive layers 106 b and 126 b are formedusing a light-transmitting material as shown in this embodiment, theaperture ratio is not reduced even when the area of the region where theconductive layers overlap with each other is increased to increase thecapacitance value. That is, the object of increasing the capacitancevalue can be achieved without reduction in aperture ratio.

In this embodiment, the conductive layers 106 a, 106 b, and 126 b areformed so that the conductive layer 106 a functioning as the sourceelectrode and the conductive layer 106 b functioning as the drainelectrode overlap with part of the conductive layer 126 a functioning asthe gate electrode. Alternatively, in the case where the conductivity ofpart of the semiconductor layer 118 a can be increased, a structure maybe employed in which the conductive layer 106 a or the conductive layer106 b does not overlap with the conductive layer 126 a (see FIGS. 9A and9B). In that case, the conductivity of at least a region 160 in whichthe conductive layer 106 a or the conductive layer 106 b does notoverlap with the conductive layer 126 a is increased. In FIGS. 9A and9B, the region 160 corresponds to a region of the semiconductor layer118 a, which is adjacent to the conductive layer 106 a or the conductivelayer 106 b. Note that the region 160 may overlap or does not have tooverlap with the conductive layer 126 a. Moreover, the region 160preferably overlaps with the conductive layer 106 a or the conductivelayer 106 b; however, this embodiment is not limited thereto.

As a method for increasing the conductivity of the region 160 in thecase where an oxide semiconductor material is used for the semiconductorlayer 118 a, a method where hydrogen is selectively added is used, forexample. When an oxide semiconductor material is not used for thesemiconductor layer, a method for increasing the conductivity may beselected depending on a material for the semiconductor layer. Forexample, when the semiconductor layer 118 a is formed using asilicon-based material, an impurity element imparting givenconductivity, such as phosphorus or boron, may be added.

With the structure in which the conductive layer 106 a or the conductivelayer 106 b does not overlap with the conductive layer 126 a in such amanner, the storage capacitance caused by the overlap between theconductive layer 106 a (or the conductive layer 106 b) and theconductive layer 126 a can be reduced. In other words, the object ofimproving the characteristics of the semiconductor device can beachieved.

Note that the addition of hydrogen can be performed after any of thefollowing steps, for example: the step of forming the semiconductorlayer 114, the step of forming the semiconductor layer 118 a, the stepof forming the insulating layer 120, or the step of forming theconductive layer 126 a. For example, when hydrogen is added after thesemiconductor layer 118 is formed, a resist mask 170 is selectivelyformed over the semiconductor layer 118 a (see FIG. 34A), and hydrogen190 is added (see FIG. 34B), whereby the region 160 can be formed (seeFIG. 34C). In that case, the semiconductor device can have a structureillustrated in FIG. 35A or FIG. 35B. This is because the conductivity ofthe region 160 is increased, so that the need for additional provisionof the conductive layer 106 b or the like is reduced. FIG. 35Aillustrates a structure in which the conductive layer 106 b is notprovided. FIG. 35B illustrates a structure in which the conductivelayers 106 a and 106 b are not provided. Note that when hydrogen isadded after the conductive layer 126 a is formed, hydrogen can be addedin a self-aligned manner by using the conductive layer 126 a as a mask.

Next, a conductive layer 128 is formed so as to cover the conductivelayers 126 a and 126 b (see FIGS. 4B1 and 4B2). The conductive layer 128can be formed using a material and a method which are similar to thoseof the conductive layer 108. Since the description of the conductivelayer 108 can be referred to for the details of the conductive layer128, the description is not repeated. In that case also, it ispreferable to form the conductive layer 108 and the conductive layer 128using the same material because reduction in cost, increase inthroughput, and the like can be achieved.

Then, a resist mask 130 is formed over the conductive layer 128, and theconductive layer 128 is selectively etched using the resist mask 130, sothat the conductive layers 132 a and 132 b are formed (see FIGS. 4C1 and4C2, refer to FIG. 1A for the conductive layer 132 b). The conductivelayer 132 a functions as the gate wiring, and the conductive layer 132 bfunctions as the capacitor wiring. Since the conductive layer 132 a isformed using a material with a light-shielding property, the conductivelayer 132 a has a function of shielding light. The resist mask 130 isremoved after the conductive layers 132 a and 132 b are formed.

Note that the steps in which the conductive layers 132 a and 132 b areformed after the conductive layers 126 a and 126 b are formed aredescribed in this embodiment; however, the disclosed invention shouldnot be construed as being limited thereto. For example, the order offormation of the conductive layers 126 a and 126 b and formation of theconductive layers 132 a and 132 b may be changed. That is, theconductive layer 126 a functioning as the gate electrode and theconductive layer 126 b functioning as the electrode of the storagecapacitor can be formed after the conductive layer 132 a functioning asthe gate wiring and the conductive layer 132 b functioning as thecapacitor wiring are formed (see FIGS. 10A and 10B). Note that althoughthe order of formation of the conductive layers 106 a and 106 b andformation of the conductive layer 112 is not changed in FIGS. 10A and10B, the order may be changed.

Note that the conductive layers 126 a and 126 b are preferably formedthinner than the conductive layer 132 a or the like. It is advantageousto make the conductive layers 126 a and 126 b thinner because, althoughthe resistance is increased, the transmittance can be further increased.It is needless to say that one embodiment of the disclosed inventionshould not be construed as being limited thereto.

Alternatively, the conductive layer 132 b may be formed so as to remainover the conductive layer 126 b (see FIGS. 11A and 11B). By forming theconductive layer 132 b in such a manner, the wiring resistance of thecapacitor wiring can be reduced. Note that it is preferable that thewidth of the conductive layer 132 b over the conductive layer 126 b besufficiently smaller than that of the conductive layer 126 b. By formingthe conductive layer 132 b in this manner, the object of reducing thewiring resistance of the capacitor wiring can be achieved withoutsubstantial reduction in aperture ratio.

Next, an insulating layer 134 is formed so as to cover the gateinsulating layer 120, the conductive layers 126 a and 126 b, and theconductive layers 132 a and 132 b (see FIGS. 4D1 and 4D2). A surface ofthe insulating layer 134 is preferably made flat because an electrode(the pixel electrode) is formed later on the surface. In particular, inone embodiment of the disclosed invention, a variety of elements can beformed using a light-transmitting material, so that a region where theseelements are formed can be used as a display region (an opening region).Accordingly, it is extremely useful to form the insulating layer 134 sothat unevenness caused by an element and a wiring is reduced.

The insulating layer 134 can be formed to have a single-layer structureor a layered structure of an insulating film formed using a materialcontaining oxygen and/or nitrogen, such as silicon oxide, siliconnitride, silicon oxynitride, or silicon nitride oxide; a film containingcarbon such as a diamond-like carbon (DLC); a film formed using anorganic material such as epoxy, polyimide, polyamide, polyvinylphenol,benzocyclobutene, or acrylic or a siloxane material such as a siloxaneresin. For example, a film containing silicon nitride is preferably usedto increase the reliability of the element because the film containingsilicon nitride is highly effective in blocking impurities. Moreover, afilm containing an organic material is preferably used to improve thecharacteristics of the element because the film containing the organicmaterial can effectively reduce unevenness. Note that when theinsulating layer 134 has a layered structure of a film containingsilicon nitride and a film containing an organic material, it ispreferable that the film containing silicon nitride be arranged on thelower side (on the side nearer to the element) in the drawing and thefilm containing the organic material be arranged on the upper side (onthe side of the surface where the pixel electrode is formed). Theinsulating layer 134 preferably has a sufficient light-transmittingproperty.

When the insulating layer 134 has a two-layer structure of an insulatinglayer 134 a and an insulating layer 134 b (see FIG. 36A), a region ofthe insulating layer 134 b, which overlaps with the conductive layer 126b, is removed by etching (see FIG. 36B), so that it is possible toincrease the capacitance value of a capacitor formed between theconductive layer 126 b and the conductive layer 140 which is formedlater (see FIG. 36C). Note that one embodiment of the disclosedinvention is not limited to the above, and the insulating layer 134 mayhave a multi-layer structure of three layers or more.

The insulating layer 134 may be formed to have a function of a colorfilter. When a color filter is thus formed over a substrate where theelement is formed, alignment in attaching a counter substrate or thelike becomes easy. It is needless to say that the insulating layer 134does not necessarily have a function of a color filter, and a layerfunctioning as a color filter may be additionally formed over thesubstrate 100. Note that in one embodiment of the disclosed invention,the source wiring, the gate wiring, and the like are formed using amaterial with a light-shielding property. Accordingly, a portion betweenpixels can be shielded from light without additionally forming a blackmask (a black matrix). That is, a high-performance semiconductor devicecan be provided while a process can be simplified as compared to thecase where a black mask is additionally formed. It is needless to saythat one embodiment of the disclosed invention should not be construedas being limited thereto, and a black mask may be additionally formed.

Note that in the case where great inconvenience is not caused withoutthe insulating layer 134, a structure in which the insulating layer 134is not formed can be employed. In that case, there is an advantage inthat a process can be simplified.

After that, a contact hole 136 which reaches the conductive layer 106 bis formed in the insulating layer 134, and part of the surface of theconductive layer 106 b is exposed (see FIGS. 5A1 and 5A2).

Then, a conductive layer 138 is formed so as to cover the insulatinglayer 134 (see FIGS. 5B1 and 5B2). Since the contact hole is formed inthe insulating layer 134, the conductive layer 106 b and the conductivelayer 138 are electrically connected to each other.

The conductive layer 138 can be formed using a material and a methodwhich are similar to those of the conductive layers 102 and 122. Sincethe description of the conductive layers 102 and 122 can be referred tofor the details of the conductive layer 138, the description is notrepeated. Note that the conductive layer 138 preferably has alight-transmitting property. In that case also, it is preferable to formthe conductive layers 102, 122, and 138 using the same material becausereduction in cost, increase in throughput, and the like can be achieved.

Next, a resist mask is formed over the conductive layer 138, and theconductive layer 138 is selectively etched using the resist mask, sothat the conductive layer 140 is formed (see FIGS. 5C1 and 5C2). Here,the conductive layer 140 functions as the pixel electrode.

Note that the conductive layer 140 is preferably formed so that an endportion of the conductive layer 140 overlaps with the conductive layer112 or the conductive layer 132 a. By forming the conductive layer 140in such a manner, the aperture ratio of a pixel can be maximized andunnecessary light leakage or the like can be reduced. Accordingly, theeffect of increasing the contrast can be obtained. That is, the objectof improving the characteristics of a display device can be achieved.

Although not illustrated, the source wiring, the source electrode, thegate wiring, the gate electrode, the capacitor wiring, the capacitorelectrode, and the like can be connected to each other by using aconductive layer formed of the conductive layer 138. In other words, theconductive layer formed of the conductive layer 138 can function as avariety of wirings.

Thus, a semiconductor device including the transistor 150 with alight-transmitting property and the storage capacitor 152 with alight-transmitting property can be formed (see FIGS. 5C1 and 5C2).

The transistor 150 and the storage capacitor 152 are formed using alight-transmitting material as described above, so that light can pass aregion where the source electrode, the drain electrode, the gateelectrode, and the like are formed; thus, the aperture ratio of a pixelcan be increased. The conductive layer functioning as the source wiring,the gate wiring, or the capacitor wiring is formed using a lowresistance material, whereby the wiring resistance can be reduced andpower consumption can be increased. Moreover, distortion of waveforms ofsignals can be reduced, and a voltage drop due to wiring resistance canbe suppressed. Further, the source wiring, the gate wiring, and the likeare formed using a material with a light-shielding property, so that aportion between pixels can be shielded from light without additionallyforming a black mask (a black matrix). That is, a high-performancesemiconductor device can be provided while a process can be simplifiedas compared to the case where a black mask is additionally formed.

In addition, the capacitor electrode is formed using alight-transmitting material, whereby the area of the capacitor electrodecan be increased sufficiently. That is, the capacitance value of thestorage capacitor can be increased sufficiently. Accordingly, apotential holding property of the pixel electrode is improved, and thedisplay quality is improved. Moreover, a feed-through potential can bereduced. Further, crosstalk can be reduced. Furthermore, flickers can bereduced.

Since the transistor 150 is formed using a material with alight-transmitting property, the degree of freedom in setting thechannel length (L) and the channel width (W) of the transistor 150 ishigh (i.e., the degree of freedom for the layout is high). This isbecause the aperture ratio is not affected by the channel length and thechannel width. Note that when the element is used for an object whichdoes not need the transmittance, such as a driver circuit, the elementmay be formed using a material without a light-transmitting property. Inthat case, an element used in a pixel portion and an element used inother regions (e.g., a driver circuit) can be separately formed.

FIGS. 37A and 37B and FIGS. 38A and 38B illustrate other examples of astructure of a semiconductor device. FIGS. 37A and 37B illustrate anexample in which the conductive layer 112 functioning as the sourcewiring has a function of the source electrode, and the conductive layer132 a functioning as the gate wiring has a function of the gateelectrode. Here, the conductive layers 112 and 132 a can be formed usinga material with high conductivity. On the other hand, the conductivelayer 106 b functioning as the drain electrode is preferably formedusing a material with a light-transmitting property. Note that aconductive layer 180 functioning as the capacitor wiring may be formedusing a material with high conductivity or a material with alight-transmitting property. FIGS. 38A and 38B illustrate an example inwhich the conductive layer 126 a functioning as the gate electrode has afunction of one electrode of the storage capacitor. That is, aconductive layer functioning as a gate wiring of the previous stage orthe next stage (corresponding to the conductive layer 132 a) has afunction of the capacitor wiring. Here, a conductive layer 182 formed inthe same step as the conductive layer 106 a or the conductive layer 106b has a function of the other electrode of the storage capacitor. Sincethe conductive layer 182 is formed in a region overlapping with thepixel portion, the conductive layer 182 is preferably formed using amaterial with a light-transmitting property.

Note that the channel length (L) and the channel width (W) of thetransistor can be larger than the width of the conductive layer 132 a orthe like. This is because the semiconductor layer 118 a is formed usinga light-transmitting material, so that the aperture ratio does notdepend on the size of the semiconductor layer 118 a. However, oneembodiment of the disclosed invention is not construed as being limitedthereto. A plurality of transistors may be arranged in series or inparallel. Accordingly, the number of transistors can be increased.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, another example of a method for manufacturing asemiconductor device will be described with reference to FIGS. 12A and12B, FIGS. 13A1 to 13D1 and 13A2 to 13D2, FIGS. 14A1 to 14C1 and 14A2 to14C2, FIGS. 15A1 to 15C1 and 15A2 to 15C2, and FIGS. 16A1 to 16C1 and16A2 to 16C2. Note that the method for manufacturing the semiconductordevice according to this embodiment has a lot in common with themanufacturing method according to Embodiment 1. Therefore, in thefollowing description, the description of the same structures, the samereference numerals, and the like are not repeated.

FIGS. 12A and 12B illustrate an example of a structure of asemiconductor device according to this embodiment. In the structure inFIGS. 12A and 12B, the conductive layers with a light-transmittingproperty (e.g., the conductive layers 106 a, 126 a, and 126 b) areplaced below the conductive layers with a light-shielding property(e.g., the conductive layers 112, 132 a, and 132 b) (see FIGS. 12A and12B). FIG. 12A is a plan view, and FIG. 12B is a cross-sectional viewalong A-B in FIG. 12A.

Next, an example of a method for manufacturing the semiconductor devicewill be described.

First, the conductive layer 102 and the conductive layer 108 aresequentially stacked over the substrate 100 having the insulatingsurface (see FIGS. 13A1 and 13A2). It is possible to refer to Embodiment1 for the details of the substrate 100 having the insulating surface,the conductive layer 102, and the conductive layer 108.

Although not shown, a base film is preferably provided over thesubstrate 100 having the insulating surface. It is possible to refer toEmbodiment 1 for the details of the base film. Note that one embodimentof the disclosed invention is not limited to formation of the base film.

Next, a resist mask 105 a and a resist mask 105 b are formed over theconductive layer 108, and the conductive layers 102 and 108 areselectively etched using the resist masks 105 a and 105 b, so that theconductive layer 106 a, the conductive layer 106 b, a conductive layer109 a, and a conductive layer 109 b are formed (see FIGS. 13B1 and13B2).

One of the differences between the method for manufacturing thesemiconductor device according to this embodiment and that according toEmbodiment 1 is an etching step of the conductive layer 102 and theconductive layer 108. In this embodiment, the resist masks 105 a and 105b used in the etching step are formed using a multi-tone mask.

A multi-tone mask is a mask capable of light exposure with multi-levelamount of light. With the use of a multi-tone mask, light exposure isperformed with three levels of light amount to provide an exposedregion, a half-exposed region, and an unexposed region. That is, amulti-tone photomask makes it possible to form a resist mask with pluralthicknesses (typically, two levels of thicknesses) by one-time exposureand development. Accordingly, by using a multi-tone mask, the number ofphotomasks to be used can be reduced.

Typical examples of a multi-tone mask are a gray-tone mask and ahalf-tone mask. The gray-tone mask includes a light-shielding portionformed over a light-transmitting substrate by using a material layerwith a light-shielding property, and a slit portion provided in thematerial layer with a light-shielding property. The slit portion hasslits (including dots, meshes, and the like) that are provided atintervals which are less than or equal to the resolution limit of lightused for light exposure, so that the slit portion has a function ofcontrolling the light transmittance. Note that the slit portion can haveslits at regular intervals or irregular intervals. The half-tone maskincludes a light-shielding portion formed over a light-transmittingsubstrate by using a material layer with a light-shielding property, anda semi-transmissive portion formed using a material layer with apredetermined light-transmitting property. The semi-transmissive portionhas light transmittance depending on a material and the thickness of thematerial layer. The light transmittance of the semi-transmissive portionis approximately in the range of 10% to 70%.

FIGS. 17A1 and 17B1 each illustrate a cross section of a typicalmulti-tone mask. FIG. 17A1 shows a gray-tone mask 400. FIG. 17B1 shows ahalf-tone mask 410.

The gray-tone mask 400 illustrated in FIG. 17A1 includes alight-shielding portion 402 formed over a light-transmitting substrate401 by using a material layer with a light-shielding property, and aslit portion 403 formed using patterns of the material layer with alight-shielding property.

The slit portion 403 has slits provided at intervals which are less thanor equal to the resolution limit of light used for light exposure. Forthe light-transmitting substrate 401, a quartz substrate or the like canbe used. The light-shielding layer forming the light-shielding portion402 and the slit portion 403 may be formed using a metal film, and ispreferably formed using chromium, chromium oxide, or the like. Whenlight is emitted to the gray-tone mask 400 illustrated in FIG. 17A1, thelight transmittance illustrated in FIG. 17A2 is obtained.

The half-tone mask 410 illustrated in FIG. 17B1 includes alight-shielding portion 412 formed over a light-transmitting substrate411 by using a material layer with a light-shielding property, and asemi-transmissive portion 413 formed using a material layer with apredetermined light-transmitting property.

The semi-transmissive portion 413 can be formed using a material layerof MoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-shieldingportion 412 may be formed using a material similar to that for thelight-shielding portion of the gray-tone mask. Note that in FIG. 17B1,the light-shielding portion 412 has a layered structure of the materiallayer with a predetermined light-transmitting property and the materiallayer with a light-shielding property. When light is emitted to thehalf-tone mask 410 illustrated in FIG. 17B1, the light transmittanceillustrated in FIG. 17B2 is obtained.

Light exposure and development are performed using the above-describedmulti-tone mask, whereby the resist mask 105 a having regions withdifferent thicknesses can be formed.

Wet etching or dry etching may be used for etching of the conductivelayers 102 and 108. Note that at this stage, both the conductive layer102 and the conductive layer 108 need to be etched. This etchingdetermines the shape of the conductive layer 106 a, which functions asthe source electrode of the transistor, and the conductive layer 106 b,which functions as the drain electrode of the transistor and theelectrode of the storage capacitor.

Next, the resist mask 105 a is recessed to form a resist mask 111 andthe resist mask 105 b is removed, and the conductive layer 109 a isselectively etched using the resist mask 111 to form the conductivelayer 112 and the conductive layer 109 b is removed (see FIGS. 13C1 and13C2). An example of a method for recessing the resist mask 105 a (and amethod for removing the resist mask 105 b) is ashing treatment usingoxygen plasma. Note that the method is not limited thereto.

Either wet etching or dry etching may be used for etching the conductivelayer 109 a and removing the conductive layer 109 b. Note that at thisstage, etching is performed in a condition that high selectivity of theconductive layer 109 a (the conductive layer 109 b) to the conductivelayer 106 a (the conductive layer 106 b) is obtained. In other words, itis important that the shape of the conductive layers 106 a and 106 b benot changed much by the etching. This etching determines the shape ofthe conductive layer 112, which functions as the source wiring of thetransistor. Here, since the conductive layer 112 is formed using amaterial with a light-shielding property, the conductive layer 112 has afunction of shielding light.

Note that the resist mask 111 is removed after the etching. In order toimprove the coverage of the above conductive layers with an insulatinglayer and the like which are formed later and prevent disconnection, itis preferable to form the conductive layers with their end portionstapered. By forming the conductive layers to be tapered in such amanner, the object of increasing the yield of the semiconductor devicecan be achieved. Eventually, increase in manufacturing costs of thesemiconductor device can be suppressed.

Alternatively, when the conductive layer 112 is formed by etching of theconductive layer 109 a, a conductive layer may be formed in a regionwhere a contact hole is formed later (which corresponds to FIGS. 7A and7B in Embodiment 1). By employing such a structure, the region where acontact hole is formed can be shielded from light. Accordingly, displaydefects due to unevenness of a surface of an electrode (a pixelelectrode) in a contact region can be reduced, whereby the contrast canbe increased. That is, the object of improving display characteristicscan be achieved. Note that although this structure is particularlyeffective in a liquid crystal display device, it is needless to say thatthis structure can be applied to other semiconductor devices. In thatcase, a conductive layer may be formed as appropriate in a region wherelight needs to be blocked.

Next, the semiconductor layer 114 is formed so as to cover at least theconductive layers 106 a and 106 b (see FIGS. 13D1 and 13D2). In thisembodiment, the semiconductor layer 114 is formed over the substrate 100so as to cover the conductive layers 106 a, 106 b, and 112. It ispossible to refer to Embodiment 1 for the details of the semiconductorlayer 114.

Before the semiconductor layer 114 is formed, plasma treatment may beperformed on a surface over which the semiconductor layer 114 is to beformed (e.g., surfaces of the conductive layers 106 a and 106 b and, inthe case where the base film is formed, a surface of the base film).With the plasma treatment, dust attached to the surface over which thesemiconductor layer 114 is to be formed can be removed. Moreover, byforming the semiconductor layer 114 without exposure to air after theplasma treatment is performed, the conductive layers 106 a and 106 b canbe electrically connected to the semiconductor layer 114 in a favorablemanner. In other words, the object of increasing the yield and thereliability of the semiconductor device can be achieved.

Next, the resist mask 116 a and the resist mask 116 b are formed overthe semiconductor layer 114, and the semiconductor layer 114 isselectively etched using the resist masks 116 a and 116 b, so that thesemiconductor layer 118 a and the semiconductor layer 118 b are formed(see FIGS. 14A1 and 14A2). It is possible to refer to Embodiment 1 forthe details of this step.

After that, heat treatment at 200° C. to 600° C., typically 300° C. to500° C., is preferably performed. Here, heat treatment is performed at350° C. for an hour in a nitrogen atmosphere. With the heat treatment,semiconductor characteristics of the semiconductor layers 118 a and 118b can be improved. Note that there is no particular limitation on thetiming of the heat treatment as long as the heat treatment is performedafter the semiconductor layers 118 a and 118 b are formed.

Next, the gate insulating layer 120 is formed so as to cover thesemiconductor layers 118 a and 118 b (see FIGS. 14B1 and 14B2). It ispossible to refer to Embodiment 1 for the details of the gate insulatinglayer 120.

Then, the conductive layer 122 and the conductive layer 128 aresequentially stacked over the gate insulating layer 120 (see FIGS. 14C1and 14C2). It is possible to refer to Embodiment 1 for the details ofthe conductive layers 122 and 128.

Although not illustrated, a base film is preferably provided over thesubstrate 100 having the insulating surface. It is possible to refer toEmbodiment 1 for the details of the base film.

Next, a resist mask 117 a and a resist mask 117 b are formed over theconductive layer 128, and the conductive layers 122 and 128 areselectively etched using the resist masks 117 a and 117 b, so that theconductive layers 126 a and 126 b and conductive layers 129 a and 129 bare formed (see FIGS. 15A1 and 15A2).

One of the differences between the method for manufacturing thesemiconductor device according to this embodiment and that according toEmbodiment 1 is an etching step of the conductive layer 122 and theconductive layer 128. In this embodiment, the resist masks 117 a and 117b used in the etching step are formed using a multi-tone mask. It ispossible to refer to the description of the resist masks 105 a and 105 bfor the details of the multi-tone mask and the like.

Light exposure and development are performed using the multi-tone mask,whereby the resist mask 117 a having regions with different thicknessescan be formed.

Wet etching or dry etching may be used for etching of the conductivelayers 122 and 128. Note that at this stage, both the conductive layer122 and the conductive layer 128 need to be etched. This etchingdetermines the shape of the conductive layer 126 a, which functions asthe gate electrode of the transistor, and the conductive layer 126 b,which functions as the electrode of the storage capacitor.

Next, the resist mask 117 a is recessed to form a resist mask 131 andthe resist mask 117 b is removed, and the conductive layer 129 a isselectively etched using the resist mask 131 to form the conductivelayers 132 a and 132 b and the conductive layer 129 b is removed (seeFIGS. 15B1 and 15B2, refer to FIG. 12A for the conductive layer 132 b).It is possible to refer to the description of the method for recessingthe resist mask 105 a (and the method for removing the resist mask 105b) and the description of etching of the conductive layer 109 a (removalof the conductive layer 109 b) for the details of a method for recessingthe resist mask 117 a (and a method for removing the resist mask 117 b)and etching of the conductive layer 129 a (removal of the conductivelayer 129 b). Note that at this stage, etching is performed in acondition that high selectivity of the conductive layer 129 a (theconductive layer 129 b) to the conductive layer 126 a (the conductivelayer 126 b) is obtained. In other words, it is important that the shapeof the conductive layers 126 a and 126 b be not changes much by theetching. This etching determines the shape of the conductive layer 132a, which functions as the gate wiring of the transistor, and theconductive layer 132 b, which functions as a wiring of the storagecapacitor. Here, since the conductive layer 132 a is formed using amaterial with a light-shielding property, the conductive layer 132 a hasa function of shielding light.

Note that the resist mask 131 is removed after the etching. In order toimprove the coverage of the above conductive layers with an insulatinglayer and the like which are formed later and prevent disconnection, itis preferable to form the conductive layers with their end portionstapered. By forming the conductive layers to be tapered in such amanner, the object of increasing the yield of the semiconductor devicecan be achieved.

Note that the area of a region where the conductive layer 106 b and theconductive layer 126 b overlap with each other can be changed asappropriate. Since the conductive layers 106 b and 126 b are formedusing a light-transmitting material as shown in this embodiment, theaperture ratio is not reduced even when the area of the region where theconductive layers overlap with each other is increased to increase thecapacitance value. That is, the object of increasing the capacitancevalue can be achieved without reduction in aperture ratio.

In this embodiment, the conductive layers 106 a, 106 b, and 126 b areformed so that the conductive layer 106 a functioning as the sourceelectrode and the conductive layer 106 b functioning as the drainelectrode overlap with part of the conductive layer 126 a functioning asthe gate electrode. Alternatively, in the case where the conductivity ofpart of the semiconductor layer 118 a can be increased, a structure maybe employed in which the conductive layer 106 a or the conductive layer106 b does not overlap with the conductive layer 126 a (whichcorresponds to FIGS. 9A and 9B in Embodiment 1). It is possible to referto Embodiment 1 for the details. With the structure in which theconductive layer 106 a or the conductive layer 106 b does not overlapwith the conductive layer 126 a in such a manner, the storagecapacitance caused by the overlap between the conductive layer 106 a (orthe conductive layer 106 b) and the conductive layer 126 a can bereduced. In other words, the object of improving the characteristics ofthe semiconductor layer can be achieved.

Alternatively, the conductive layer 132 b may be formed so as to remainover the conductive layer 126 b (which corresponds to FIGS. 11A and 11Bin Embodiment 1). By forming the conductive layer 132 b in such amanner, the wiring resistance of the capacitor wiring can be reduced.Note that it is preferable that the width of the conductive layer 132 bover the conductive layer 126 b be sufficiently smaller than that of theconductive layer 126 b. By forming the conductive layer 132 b in thismanner, the object of reducing the wiring resistance of the capacitorwiring can be achieved without substantial reduction in aperture ratio.

Next, the insulating layer 134 is formed so as to cover the gateinsulating layer 120, the conductive layers 126 a and 126 b, and theconductive layers 132 a and 132 b (see FIGS. 15C1 and 15C2). It ispossible to refer to Embodiment 1 for the details of the insulatinglayer 134.

Note that in the case where great inconvenience is not caused withoutthe insulating layer 134, a structure in which the insulating layer 134is not formed can be employed. In that case, there is an advantage inthat a process can be simplified.

After that, the contact hole 136 which reaches the conductive layer 106b is formed in the insulating layer 134, and part of the surface of theconductive layer 106 b is exposed (see FIGS. 16A1 and 16A2).

Then, the conductive layer 138 is formed so as to cover the insulatinglayer 134 (see FIGS. 16B1 and 16B2). Since the contact hole is formed inthe insulating layer 134, the conductive layer 106 b and the conductivelayer 138 are electrically connected to each other. It is possible torefer to Embodiment 1 for the details of the conductive layer 138.

Next, a resist mask is formed over the conductive layer 138, and theconductive layer 138 is selectively etched using the resist mask, sothat the conductive layer 140 is formed (see FIGS. 16C1 and 16C2). Here,the conductive layer 140 functions as the pixel electrode. It ispossible to refer to Embodiment 1 for the details of the conductivelayer 140 and the like.

Thus, a semiconductor device including the transistor 150 with alight-transmitting property and the storage capacitor 152 with alight-transmitting property can be formed (see FIGS. 16C1 and 16C2).

Note that in this embodiment, the wirings and the electrodes are formedusing a multi-tone mask; however, one embodiment of the disclosedinvention is not construed as being limited thereto. A multi-tone maskmay be used in either one of the step of forming the conductive layers106 a and 112 and the step of forming the conductive layers 126 a and132 a.

In this embodiment, a resist mask is formed using a multi-tone mask toperform etching. Accordingly, the number of photomasks to be used can bereduced, and the number of steps can be reduced. That is, the object ofreducing manufacturing costs of the semiconductor device can beachieved.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, another example of a semiconductor device will bedescribed with reference to FIGS. 18A and 18B, FIGS. 19A and 19B, FIGS.20A and 20B, and FIGS. 21A and 21B. Note that the semiconductor deviceaccording to this embodiment has a lot in common with the semiconductordevice according to Embodiment 1. Therefore, in the followingdescription, the description of the same structures, the same referencenumerals, and the like are not repeated.

FIGS. 18A and 18B illustrate an example of a structure of asemiconductor device according to this embodiment. The structure ispreferably used particularly for an electroluminescent display device(an EL display device); however, the disclosed invention is not limitedthereto. FIG. 18A is a plan view, and FIG. 18B is a cross-sectional viewalong E-F in FIG. 18A.

The semiconductor device illustrated in FIG. 18A includes a pixelportion including the conductive layer 112 functioning as a sourcewiring; a conductive layer 162 which is formed in a manner similar tothat of the conductive layer 112 and functions as a power supply wiring;the conductive layer 132 a which intersects the conductive layer 112 andthe conductive layer 162, and functions as a gate wiring; the transistor150 near the intersection of the conductive layer 132 a and theconductive layer 112; a transistor 154 electrically connected to theconductive layer 162; and a storage capacitor 156 electrically connectedto the conductive layer 162 (see FIGS. 18A and 18B). Note that in FIG.18A, the conductive layer 112 and the conductive layer 162 intersect theconductive layer 132 a at 90°; however, the disclosed invention is notlimited to this structure.

The transistor 150 is a so-called top-gate transistor including theconductive layer 106 a functioning as a source electrode, the conductivelayer 106 b functioning as a drain electrode, the semiconductor layer118 a, the gate insulating layer 120, and the conductive layer 126 afunctioning as a gate electrode (see FIGS. 18A and 18B). Similarly, thetransistor 154 includes a conductive layer 106 c functioning as a sourceelectrode, a conductive layer 106 d functioning as a drain electrode, asemiconductor layer 118 c, the gate insulating layer 120, and aconductive layer 126 c functioning as a gate electrode. The storagecapacitor 156 includes a conductive layer 106 e, the gate insulatinglayer 120, and the conductive layer 126 c. Note that in the abovedescription, the terms “source electrode” and “drain electrode” are usedonly for convenience.

Here, the conductive layer 112 and the conductive layer 106 a areelectrically connected to each other, and in a connection portion 158,the conductive layer 106 b and the conductive layer 126 c areelectrically connected through a conductive layer 142 (see FIGS. 18A and18B). Moreover, the conductive layer 162 and the conductive layer 106 care electrically connected to each other; the conductive layer 106 d andthe conductive layer 140 are electrically connected to each other; andthe conductive layer 162 and the conductive layer 106 e are electricallyconnected to each other. Note that the conductive layer 140 functioningas a pixel electrode and the conductive layer 142 can be formed throughthe same step. Further, a contact hole for connecting the conductivelayer 106 d and the conductive layer 140, a contact hole for connectingthe conductive layer 106 b and the conductive layer 142, and a contacthole for connecting the conductive layer 126 c and the conductive layer142 can be formed through the same step.

The conductive layer 106 a, the conductive layer 106 b, thesemiconductor layer 118 a, and the conductive layer 126 a which areincluded in the transistor 150; the conductive layer 106 c, theconductive layer 106 d, the semiconductor layer 118 c, and theconductive layer 126 c which are included in the transistor 154; and theconductive layer 106 e included in the storage capacitor 156 are formedusing a light-transmitting material. Accordingly, the aperture ratio ofa pixel can be increased.

The conductive layer 112, the conductive layer 132 a, and the conductivelayer 162 are formed using a low resistance material. Accordingly,wiring resistance can be reduced, and power consumption can be reduced.Moreover, the conductive layer 112, the conductive layer 132 a, and theconductive layer 162 are formed using a material with a light-shieldingproperty. Thus, a portion between pixels can be shielded from light.

Note that the above is the description of the case where one pixelincludes two transistors; however, the disclosed invention is notlimited thereto. Three or more transistors can be provided in one pixel.

FIGS. 19A and 19B illustrate another example of a structure of thesemiconductor device according to this embodiment. The structure ispreferably used particularly for an electroluminescent display device(an EL display device); however, the disclosed invention is not limitedthereto. FIG. 19A is a plan view, and FIG. 19B is a cross-sectional viewalong E-F in FIG. 19A.

Basically, the structure illustrated in FIGS. 19A and 19B is similar tothe structure illustrated in FIGS. 18A and 18B. The difference betweenthe structure in FIGS. 19A and 19B and the structure in FIGS. 18A and18B is the connection portion 158. The conductive layer 106 b and theconductive layer 126 c are connected through the conductive layer 142 inFIGS. 18A and 18B, whereas the conductive layer 106 b and the conductivelayer 126 c are directly connected to each other in FIGS. 19A and 19B.Since the conductive layer 142 is not necessary in that case, the sizeof the conductive layer 140 functioning as the pixel electrode can befurther increased; thus, the aperture ratio can be increased as comparedto that in the structure illustrated in FIGS. 18A and 18B. In order torealize electrical connection between the conductive layer 106 b and theconductive layer 126 c, a contact hole needs to be formed in the gateinsulating layer 120 before the conductive layer 126 c is formed.

FIGS. 20A and 20B illustrate another example of a structure of thesemiconductor device according to this embodiment. The structure ispreferably used for a display device; however, the disclosed inventionis not limited thereto. FIG. 20A is a plan view, and FIG. 20B is across-sectional view along A-B in FIG. 20A.

Basically, the structure illustrated in FIGS. 20A and 20B is similar tothe structure illustrated in FIGS. 1A and 1B. The difference between thestructure in FIGS. 20A and 20B and the structure in FIGS. 1A and 1B isthe shape of the conductive layer 106 a functioning as the sourceelectrode and the conductive layer 106 b functioning as the drainelectrode. Specifically, in the structure illustrated in FIGS. 20A and20B, the conductive layers 106 a and 106 b are formed so that a channelformation region is U-shaped. Accordingly, the channel width (W) can beincreased even when a transistor with the same area is formed. Note thatthe shape of a channel formation region is not limited to a U-shape andcan be changed as appropriate depending on desired channel width.

FIGS. 21A and 21B illustrate another example of a structure of thesemiconductor device according to this embodiment. The structure ispreferably used for a display device; however, the disclosed inventionis not limited thereto. FIG. 21A is a plan view, and FIG. 21B is across-sectional view along A-B in FIG. 21A.

The structure illustrated in FIGS. 21A and 21B is similar to thestructure illustrated in FIGS. 1A and 1B. The structure in FIGS. 21A and21B is different from the structure in FIGS. 1A and 1B in that theconductive layer 132 a functioning as the gate wiring also serves as thegate electrode (see FIGS. 21A and 21B). That is, a conductive layercorresponding to the conductive layer 126 a is not provided in FIGS. 21Aand 21B. Since the conductive layer 132 a can be formed using a lowresistance material, electric fields applied to the semiconductor layer118 a can be more uniform than that of the case where the conductivelayer 126 a (a conductive layer formed using a light-transmittingmaterial) is used for the gate electrode. Accordingly, elementcharacteristics of the transistor 150 can be improved.

Note that in FIGS. 21A and 21B, although a structure in which theconductive layer 126 a is not provided is employed, the disclosedinvention is not limited to this structure. The conductive layer 126 aelectrically connected to the conductive layer 132 a may be provided.Further, the conductive layer 106 a is formed in FIGS. 21A and 21B;alternatively, the conductive layer 112 may also serve as the conductivelayer 106 a without the formation of the conductive layer 106 a. This isbecause the conductive layer having a function of the source electrodeis formed below the conductive layer functioning as the gate wiring, sothat the need for formation of the conductive layer functioning as thesource electrode by using a light-transmitting material is reduced. Inthat case, at least the conductive layer 106 b and the conductive layer126 b are formed using a light-transmitting material.

In addition, it is needless to say that the structure according to thisembodiment can be employed in the case where a multi-tone mask is used.When a multi-tone mask is used, the conductive layer 126 a is formedbelow the conductive layer 132 a.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 4

In this embodiment, the case where a thin film transistor is formed andused in a pixel portion and a peripheral circuit portion (e.g., a drivercircuit) to manufacture a semiconductor device (a display device) havinga display function will be described. When part of or all the peripheralcircuit portion is formed over a substrate where the pixel portion isformed, a system-on-panel can be realized.

A display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement), a light-emitting element (also referred to as a light-emittingdisplay element), or the like can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Further, a display medium whose contrast is changed by anelectric effect, such as electronic ink, may be used.

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC including a controller orthe like is mounted on the panel. Moreover, an element substrate whichforms a display device is provided with a means for supplying current tothe display element in each of pixels. Specifically, the elementsubstrate may be in a state after only a pixel electrode of the displayelement is formed, or in a state after a conductive layer to be a pixelelectrode is formed and before the conductive layer is etched.

In this embodiment, an example of a liquid crystal display device isdescribed below. FIGS. 22A1 and 22A2 are plan views and FIG. 22B is across-sectional view of a panel in which thin film transistors 4010 and4011 and a liquid crystal element 4013 that are formed over a firstsubstrate 4001 are sealed by a second substrate 4006 and a sealingmaterial 4005. Here, each of FIGS. 22A1 and 22A2 is a plan view, andFIG. 22B is a cross-sectional view taken along M-N of FIGS. 22A1 and22A2.

The sealing material 4005 is provided so as to surround a pixel portion4002 and a scan line driver circuit 4004 which are provided over thefirst substrate 4001. The second substrate 4006 is provided over thepixel portion 4002 and the scan line driver circuit 4004. In otherwords, the pixel portion 4002 and the scan line driver circuit 4004 aresealed together with a liquid crystal layer 4008, by the first substrate4001, the sealing material 4005, and the second substrate 4006. Further,a signal line driver circuit 4003 that is formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared is mounted in a region different from the regionsurrounded by the sealing material 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used as appropriate.FIG. 22A1 illustrates an example where the signal line driver circuit4003 is mounted by a COG method. FIG. 22A2 illustrates an example wherethe signal line driver circuit 4003 is mounted by a TAB method.

In addition, the pixel portion 4002 and the scan line driver circuit4004, which are provided over the first substrate 4001, each include aplurality of thin film transistors. FIG. 22B illustrates the thin filmtransistor 4010 included in the pixel portion 4002 and the thin filmtransistor 4011 included in the scan line driver circuit 4004. Aninsulating layer 4020 is provided over the thin film transistors 4010and 4011.

As the thin film transistors 4010 and 4011, the thin film transistorswhich are described in the foregoing Embodiments or the like can beemployed. Note that in this embodiment, the thin film transistors 4010and 4011 are n-channel transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. The liquid crystal element 4013 is formed usingthe pixel electrode layer 4030, the counter electrode layer 4031, andthe liquid crystal layer 4008. Note that the pixel electrode layer 4030and the counter electrode layer 4031 are provided with an insulatinglayer 4032 and an insulating layer 4033, respectively, each of whichfunctions as an alignment film. The liquid crystal layer 4008 issandwiched between the pixel electrode layer 4030 and the counterelectrode layer 4031 with the insulating layers 4032 and 4033therebetween.

Note that for the first substrate 4001 and the second substrate 4006,glass, metal (typically, stainless steel), ceramic, plastics, or thelike can be used. As plastics, an FRP (fiberglass-reinforced plastics)substrate, a PVF (polyvinyl fluoride) film, a polyester film, an acrylicresin film, or the like can be used. Moreover, a sheet in which aluminumfoil is sandwiched between PVF films or polyester films can also beused.

A columnar spacer 4035 is provided in order to control the distance (acell gap) between the pixel electrode layer 4030 and the counterelectrode layer 4031. The columnar spacer 4035 can be obtained byselective etching of an insulating film. Note that a spherical spacermay be used instead of a columnar spacer. The counter electrode layer4031 is electrically connected to a common potential line provided overthe substrate where the thin film transistor 4010 is formed. Forexample, the counter electrode layer 4031 can be electrically connectedto the common potential line through conductive particles providedbetween the pair of substrates. Note that the conductive particles arepreferably contained in the sealing material 4005.

Alternatively, a liquid crystal showing a blue phase for which analignment film is unnecessary may be used. A blue phase is one of theliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperatures, a liquid crystal compositioncontaining a chiral agent of 5 wt % or more is preferably used. Thus,the temperature range can be improved. The liquid crystal compositionwhich includes a liquid crystal showing a blue phase and a chiral agenthas a small response time of 10 μs to 100 μs, has optical isotropy,which makes the alignment process unneeded, and has small viewing angledependence.

Although an example of a transmissive liquid crystal display device isdescribed in this embodiment, this embodiment is not limited thereto,and a reflective liquid crystal display device or a transflective liquidcrystal display device may be used.

As the example of the liquid crystal display device described in thisembodiment, a polarizing plate is provided on the outer surface of thesubstrate (on the viewer side) and a coloring layer and an electrodelayer used for a display element are provided on the inner surface ofthe substrate; however, the polarizing plate may be provided on theinner surface of the substrate. Moreover, the layered structure of thepolarizing plate and the coloring layer is not limited to that describedin this embodiment and may be set as appropriate depending on materialsor conditions of manufacturing steps of the polarizing plate and thecoloring layer. Further, a black mask (a black matrix) may be providedas a light-shielding film.

In this embodiment, the thin film transistor obtained in the foregoingEmbodiments is covered with the insulating layer 4020 in order to reducethe surface roughness of the thin film transistor; however, thedisclosed invention is not limited to this structure.

For the insulating layer 4020, an organic material having heatresistance, such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy, can be used. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4020 may be formed by stacking a plurality of insulating films formed ofthese materials.

Here, the siloxane-based resin corresponds to a resin including aSi—O—Si bond which is formed using a siloxane-based material as astarting material. As a substituent, an organic group (e.g., an alkylgroup or an aryl group) or a fluoro group may be used. The organic groupmay include a fluoro group.

There is no particular limitation on the method for forming theinsulating layer 4020. The insulating layer 4020 can be formed,depending on the material, by sputtering, SOG, spin coating, dipping,spray coating, droplet discharging (e.g., ink-jet, screen printing, oroffset printing), doctor knife, roll coater, curtain coater, knifecoater, or the like.

For the pixel electrode layer 4030 and the counter electrode layer 4031,the following light-transmitting conductive material can be used, forexample: indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide (alsoreferred to as ITO), indium zinc oxide, or indium tin oxide to whichsilicon oxide is added.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) may be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of 1.0×10⁴ ohm/square or less and a light transmittance of70% or more at a wavelength of 550 nm. Furthermore, the resistivity ofthe conductive high molecule contained in the conductive composition ispreferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called t-electron conjugatedconductive polymer can be used. Examples of the conductive high moleculeare polyaniline and its derivatives, polypyrrole and its derivatives,polythiophene and its derivatives, and copolymers of two or more kindsof these materials.

A variety of signals are supplied from an FPC 4018 to the signal linedriver circuit 4003, the scan line driver circuit 4004, the pixelportion 4002, or the like.

A connection terminal electrode 4015 is formed from the same conductivefilm as the pixel electrode layer 4030 included in the liquid crystalelement 4013. A terminal electrode 4016 is formed from the sameconductive film as source and drain electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 22A1, 22A2, and 22B illustrate the example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be formed separately andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be formed separately and then mounted.

FIG. 23 illustrates an example in which a TFT substrate 2600 is used fora liquid crystal display module which is one embodiment of thesemiconductor device.

In FIG. 23, the TFT substrate 2600 and a counter substrate 2601 arebonded to each other by a sealing material 2602; and an element layer2603 including a TFT and the like, a liquid crystal layer 2604 includingan alignment film and a liquid crystal, a coloring layer 2605, and thelike are provided between the TFT substrate 2600 and the countersubstrate 2601, whereby a display region is formed. The coloring layer2605 is necessary for color display. In the case of the RGB system,coloring layers corresponding to colors of red, green, and blue areprovided for respective pixels. A polarizing plate 2606, a polarizingplate 2607, and a diffuser plate 2613 are provided outside the TFTsubstrate 2600 and the counter substrate 2601. A light source includes acold cathode tube 2610 and a reflective plate 2611. A circuit board 2612is connected to a wiring circuit portion 2608 of the TFT substrate 2600through a flexible wiring board 2609. Accordingly, an external circuitsuch as a control circuit or a power source circuit is included in aliquid crystal module. Moreover, a retardation plate may be providedbetween the polarizing plate and the liquid crystal layer.

For a method for driving a liquid crystal, a TN (twisted nematic) mode,an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode,an MVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above steps, a high-performance liquid crystal displaydevice can be manufactured. This embodiment can be implemented incombination with any of the other embodiments as appropriate.

Embodiment 5

In this embodiment, active matrix electronic paper which is an exampleof a semiconductor device will be described with reference to FIG. 24. Athin film transistor 650 used for the semiconductor device can bemanufactured in a manner similar to that of the thin film transistordescribed in the foregoing Embodiments.

FIG. 24 illustrates an example of electronic paper using a twisting balldisplay system. The twisting ball display system refers to a method inwhich spherical particles each colored in black and white are arrangedbetween a first electrode layer and a second electrode layer, and apotential difference is generated between the first electrode layer andthe second electrode layer, whereby orientation of the sphericalparticles is controlled so that display is performed.

The thin film transistor 650 provided over the substrate 600 is a thinfilm transistor of the disclosed invention and has a structure in whicha semiconductor layer is sandwiched between a gate electrode layer whichis above the semiconductor layer and the source electrode layer or thedrain electrode layer which is below the semiconductor layer. Note thatthe source electrode layer or the drain electrode layer is electricallyconnected to a first electrode layer 660 through a contact hole formedin an insulating layer. A substrate 602 is provided with a secondelectrode layer 670. Spherical particles 680 each having a black region680 a and a white region 680 b are provided between the first electrodelayer 660 and the second electrode layer 670. A space around thespherical particles 680 is filled with a filler 682 such as a resin (seeFIG. 24). In FIG. 24, the first electrode layer 660 corresponds to apixel electrode, and the second electrode layer 670 corresponds to acommon electrode. The second electrode layer 670 is electricallyconnected to a common potential line provided over the substrate wherethe thin film transistor 650 is formed.

Instead of the twisting ball, an electrophoretic display element canalso be used. In that case, a microcapsule having a diameter ofapproximately 10 μm to 200 μm in which transparent liquid,positively-charged white microparticles, and negatively-charged blackmicroparticles are encapsulated is used, for example. When an electricfield is applied by the first electrode layer and the second electrodelayer, the white microparticles and the black microparticles move indirections opposite to each other, so that white or black is displayed.The electrophoretic display element has higher reflectivity than aliquid crystal display element, so that an auxiliary light isunnecessary and a display portion can be recognized in a place wherebrightness is not sufficient. In addition, there is an advantage thateven when power is not supplied to the display portion, an image whichhas been displayed once can be maintained.

Accordingly, high-performance electronic paper can be manufactured usingthe disclosed invention. Note that this embodiment can be implemented incombination with any of the other embodiments as appropriate.

Embodiment 6

In this embodiment, an example of a light-emitting display device willbe described as a semiconductor device. As a display element included ina display device, a light-emitting element utilizing electroluminescenceis described here. Light-emitting elements utilizing electroluminescenceare classified according to whether a light-emitting material is anorganic compound or an inorganic compound. In general, the former isreferred to as an organic EL element, and the latter is referred to asan inorganic EL element.

In an organic EL element, voltage is applied to the light-emittingelement, so that electrons and holes are separately injected from a pairof electrodes into a layer containing a light-emitting organic compound,and current flows. Then, the carriers (electrons and holes) arerecombined, so that light is emitted. Owing to such a mechanism, thelight-emitting element is called a current-excitation light-emittingelement.

Inorganic EL elements are classified in a dispersive inorganic ELelement and a thin-film inorganic EL element according to their elementstructures. A dispersive inorganic EL element includes a light-emittinglayer in which particles of a light-emitting material are dispersed in abinder, and light emission mechanism thereof is donor-acceptorrecombination light emission utilizing a donor level and an acceptorlevel. In a thin film inorganic EL element, a light-emitting layer issandwiched between dielectric layers, and the dielectric layers aresandwiched between electrodes. Light emission mechanism of the thin filminorganic EL element is local light emission utilizing inner-shellelectron transition of a metal ion. Note that here, an organic ELelement is used as a light-emitting element.

A structure of a light-emitting element is described with reference toFIGS. 25A to 25C. A cross-sectional structure of a pixel including ann-channel driving TFT is described as an example. TFTs 701, 711, and 721used for semiconductor devices illustrated in FIGS. 25A to 25C can bemanufactured in a manner similar to that of the thin film transistorsdescribed in the foregoing Embodiments.

In order to extract light from a light-emitting element, at least one ofan anode and a cathode is transparent. Here, the term “transparent”means that at least transmittance at the wavelength of emitted light issufficiently high. As a method for extracting light, there are a topemission method (a top extraction method) by which light is extractedfrom a side opposite to a substrate where a thin film transistor and alight-emitting element are formed, a bottom emission method (a bottomextraction method) by which light is extracted from the substrate side,a dual emission method (a dual extraction method) by which light isextracted from both the substrate side and the side opposite to thesubstrate, and the like.

A light-emitting element with a top emission method is described withreference to FIG. 25A.

FIG. 25A is a cross-sectional view of a pixel in the case where light isemitted from a light-emitting element 702 to an anode 705 side. Here,the light-emitting element 702 is formed over a light-transmittingconductive layer 707 which is electrically connected to the driving TFT701, and a light-emitting layer 704 and the anode 705 are stacked inthis order over a cathode 703. As the cathode 703, a conductive filmwhich has a low work function and reflects light can be used. Forexample, the cathode 703 is preferably formed using a material such asCa, Al, MgAg, or AlLi. The light-emitting layer 704 may be formed usinga single layer or a plurality of layers stacked. When the light-emittinglayer 704 is formed using a plurality of layers, an electron-injectinglayer, an electron-transporting layer, a light-emitting layer, ahole-transporting layer, and a hole-injecting layer are preferablystacked in this order over the cathode 703; however, it is needless tosay that it is not necessary to form all of these layers. The anode 705is formed using a light-transmitting conductive material. For example,the following light-transmitting conductive material may be used: indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (also referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A structure in which the light-emitting layer 704 is sandwiched betweenthe cathode 703 and the anode 705 can be called the light-emittingelement 702. In the case of the pixel illustrated in FIG. 25A, light isemitted from the light-emitting element 702 to the anode 705 side asindicated by an arrow. The structure of the light-emitting element 702may be a microcavity structure. Accordingly, it is possible to select awavelength to be extracted, so that the color purity can be improved.Note that in that case, the thickness of layers included in thelight-emitting element 702 is set depending on the wavelength to beextracted. Moreover, an electrode is preferably formed using a materialwith a predetermined reflectivity.

An insulating layer containing silicon nitride, silicon oxide, or thelike may be formed over the anode 705. Accordingly, deterioration of thelight-emitting element can be suppressed.

Next, a light-emitting element with a bottom emission method isdescribed with reference to FIG. 25B.

FIG. 25B is a cross-sectional view of a pixel in the case where light isemitted from a light-emitting element 712 to a cathode 713 side. Here,the cathode 713 of the light-emitting element 712 is formed over alight-transmitting conductive film 717 which is electrically connectedto the driving TFT 711, and a light-emitting layer 714 and an anode 715are stacked in this order over the cathode 713. Note that when the anode715 has a light-transmitting property, a light-shielding film 716 may beprovided so as to cover the anode 715. For the cathode 713, a conductivematerial having a low work function can be used as in FIG. 25A. Notethat the cathode 713 has a thickness that can transmit light (preferablyapproximately 5 nm to 30 nm). For example, an aluminum film with athickness of approximately 20 nm can be used as the cathode 713. As inFIG. 25A, the light-emitting layer 714 may be formed using a singlelayer or a plurality of layers stacked. The anode 715 does notnecessarily transmit light, but may be formed using a light-transmittingconductive material as in FIG. 25A. The light-shielding film 716 can beformed using a metal which reflects light or the like; however, thisembodiment is not limited thereto. Note that when the light-shieldingfilm 716 has a function of reflecting light, light extraction efficiencycan be improved.

A structure in which the light-emitting layer 714 is sandwiched betweenthe cathode 713 and the anode 715 can be called the light-emittingelement 712. In the case of the pixel illustrated in FIG. 25B, light isemitted from the light-emitting element 712 to the cathode 713 side asindicated by an arrow. The structure of the light-emitting element 712may be a microcavity structure. Moreover, an insulating layer may beformed over the anode 715.

Next, a light-emitting element with a dual emission method is describedwith reference to FIG. 25C.

In FIG. 25C, a cathode 723 of a light-emitting element 722 is formedover a light-transmitting conductive film 727 which is electricallyconnected to the driving TFT 721, and a light-emitting layer 724 and ananode 725 are stacked in this order over the cathode 723. For thecathode 723, a conductive material having a low work function can beused as in FIG. 25A. Note that the cathode 723 has a thickness that cantransmit light. For example, a 20-nm-thick aluminum film can be used asthe cathode 723. As in FIG. 25A, the light-emitting layer 724 may beformed using a single layer or a plurality of layers stacked. As in FIG.25A, the anode 725 can be formed using a light-transmitting conductivematerial.

A structure where the cathode 723, the light-emitting layer 724, and theanode 725 overlap with each another can be called the light-emittingelement 722. In the case of the pixel illustrated in FIG. 25C, light isemitted from the light-emitting element 722 to both the anode 725 sideand the cathode 723 side as indicated by arrows. The structure of thelight-emitting element 722 may be a microcavity structure. Moreover, aninsulating layer may be formed over the anode 725.

The organic EL element is described here as the light-emitting element;alternatively, it is possible to provide an inorganic EL element as thelight-emitting element. In addition, the example is shown here in whichthe thin film transistor (the driving TFT) for controlling driving ofthe light-emitting element is electrically connected to thelight-emitting element; alternatively, a structure may be employed inwhich a TFT for current control is connected between the driving TFT andthe light-emitting element.

Note that the semiconductor device described in this embodiment is notlimited to have the structures illustrated in FIGS. 25A to 25C, and canbe modified in various ways.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which corresponds toone embodiment of the semiconductor device, are described with referenceto FIGS. 26A and 26B. FIG. 26A is a plan view and FIG. 26B is across-sectional view of a panel in which thin film transistors 4509 and4510 and a light-emitting element 4511 that are formed over a firstsubstrate 4501 are sealed by a second substrate 4506 and a sealingmaterial 4505. Here, FIG. 26A is a plan view, and FIG. 26B is across-sectional view taken along H-I of FIG. 26A.

The sealing material 4505 is provided so as to surround a pixel portion4502, signal line driver circuits 4503 a and 4503 b, and scan linedriver circuits 4504 a and 4504 b, which are provided over the firstsubstrate 4501. Moreover, a second substrate 4506 is provided over thepixel portion 4502, the signal line driver circuits 4503 a and 4503 b,and the scan line driver circuits 4504 a and 4504 b. In other words, thepixel portion 4502, the signal line driver circuits 4503 a and 4503 b,and the scan line driver circuits 4504 a and 4504 b are sealed togetherwith a filler 4507, by the first substrate 4501, the sealing material4505, and the second substrate 4506. In such a manner, packaging(sealing) is preferably performed using a protective film (e.g., abonding film or an ultraviolet curable resin film), a cover material, orthe like with high air-tightness and little degasification.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b, which are formedover the first substrate 4501, each include a plurality of thin filmtransistors. FIG. 26B illustrates the thin film transistor 4510 includedin the pixel portion 4502 and the thin film transistor 4509 included inthe signal line driver circuit 4503 a.

As the thin film transistors 4509 and 4510, the thin film transistorsdescribed in the foregoing Embodiments can be employed. Note that inthis embodiment, the thin film transistors 4509 and 4510 are n-channeltransistors.

Reference numeral 4511 denotes a light-emitting element. A firstelectrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that the structure of the light-emitting element 4511 is notlimited to the layered structure shown in this embodiment, whichincludes the first electrode layer 4517, a second electrode 4512, anelectroluminescent layer 4513, and a third electrode layer 4514. Thestructure of the light-emitting element 4511 can be changed asappropriate depending on the direction in which light is extracted fromthe light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, organic polysiloxane, or the like. It is particularlypreferable that the partition 4520 be formed using a photosensitivematerial to have an opening over the first electrode layer 4517 so thata sidewall of the opening is formed as an inclined surface withcontinuous curvature.

The electroluminescent layer 4513 may be formed using a single layer ora plurality of layers stacked.

In order to prevent oxygen, hydrogen, moisture, carbon dioxide, or thelike from entering the light-emitting element 4511, a protective filmmay be formed over the third electrode layer 4514 and the partition4520. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

A variety of signals are supplied from FPCs 4518 a and 4518 b to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, the pixel portion 4502, or the like.

In this embodiment, an example is described in which a connectionterminal electrode 4515 is formed from the same conductive film as thefirst electrode layer 4517 of the light-emitting element 4511, and aterminal electrode 4516 is formed from the same conductive film as thesource and drain electrode layers of the thin film transistors 4509 and4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 should have a light-transmittingproperty. Examples of a substrate having a light-transmitting propertyare a glass plate, a plastic plate, a polyester film, and an acrylicfilm.

As the filler 4507, an ultraviolet curable resin, a thermosetting resin,or the like can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinylacetate (EVA), or the like can be used. This embodiment shows an examplewhere nitrogen is used for the filler.

If needed, a polarizing plate, a circularly polarizing plate (includingan elliptically polarizing plate), a retardation plate (a quarter-waveplate or a half-wave plate), or an optical film such as a color filtermay be provided on a light-emitting surface of the light-emittingelement. Further, an antireflection treatment may be performed on asurface. For example, anti-glare treatment may be performed by whichreflected light can be diffused by projections and depressions on thesurface so that the glare can be reduced.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared. Alternatively, only the signal line driver circuitsor part thereof or only the scan line driver circuits or part thereofmay be separately formed and mounted. This embodiment is not limited tothe structure illustrated in FIGS. 26A and 26B.

Through the above steps, a high-performance light-emitting displaydevice (display panel) can be manufactured.

Next, a structure and an operation of a pixel to which digital timeratio grayscale driving can be applied are described. FIGS. 39A and 39Beach illustrate an example of a pixel structure to which digital timeratio grayscale driving can be applied. Here, an example is described inwhich one pixel includes two n-channel transistors in which an oxidesemiconductor layer (an In—Ga—Zn—O-based non-single-crystal film) isused in a channel formation region.

In FIG. 39A, a pixel 6400 includes a switching transistor 6401, adriving transistor 6402, a light-emitting element 6404, and a capacitor6403. A gate of the switching transistor 6401 is connected to a scanline 6406. A first electrode (one of a source electrode and a drainelectrode) of the switching transistor 6401 is connected to a signalline 6405. A second electrode (the other of the source electrode and thedrain electrode) of the switching transistor 6401 is connected to a gateof the driving transistor 6402. The gate of the driving transistor 6402is connected to a power supply line 6407 through the capacitor 6403. Afirst electrode of the driving transistor 6402 is connected to the powersupply line 6407. A second electrode of the driving transistor 6402 isconnected to a first electrode (a pixel electrode) of the light-emittingelement 6404. A second electrode of the light-emitting element 6404corresponds to a common electrode 6408.

Note that as for the relation of potentials of the second electrode (onthe common electrode 6408 side) and the first electrode (on the powersupply line 6407 side) of the light-emitting element 6404, one of thepotentials can be set higher than the other. In the light-emittingdisplay device, the potential difference between a high potential and alow potential is applied to the light-emitting element 6404 and currentgenerated by the potential difference makes the light-emitting element6404 emit light; therefore, the potentials are set so that the potentialdifference between the high potential and the low potential is equal toor higher than the threshold voltage of the light-emitting element 6404.

Note that gate capacitance of the driving transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel region and the gate electrode.

In the case of using a voltage-input voltage-driving method, a videosignal which turns the driving transistor 6402 on or off is input to thegate of the driving transistor 6402. That is, the driving transistor6402 operates in a linear region.

In addition, by making input signals vary, analog grayscale driving canbe realized using the pixel structure illustrated in FIG. 39A. Forexample, when an analog video signal is used, it is possible to supplycurrent corresponding to the video signal to the light-emitting element6404 and perform analog grayscale driving. The video signal ispreferably a signal with which the driving transistor 6402 operates in asaturation region.

Further, the potential of the power supply line 6407 may be changed in apulse manner. In this case, it is preferable to employ a structureillustrated in FIG. 39B.

Furthermore, in the structure in FIG. 39A, the potential of the secondelectrode of the light-emitting element 6404 in a given pixel is oftenthe same as the potential of the second electrode in another pixel (thepotential of the common electrode 6408); alternatively, cathodes may bepatterned for each pixel and connected to their respective drivingtransistors.

Note that one embodiment of the disclosed invention is not construed asbeing limited to the pixel structures illustrated in FIGS. 39A and 39B.For example, a switch, a resistor, a capacitor, a transistor, a logiccircuit, or the like may be added to the pixel illustrated in FIGS. 39Aand 39B.

Note that this embodiment can be implemented in combination with any ofthe other embodiments as appropriate.

Embodiment 7

In this embodiment, an example in which at least part of a drivercircuit and a thin film transistor provided in a pixel portion areformed over one substrate in a display device will be described below.

FIG. 27A illustrates an example of a block diagram of an active matrixdisplay device, which is one example of the display device. The displaydevice illustrated in FIG. 27A includes, over a substrate 5300, a pixelportion 5301 including a plurality of pixels each provided with adisplay element, a scan line driver circuit 5302 for selecting a pixel,and a signal line driver circuit 5303 for controlling input of a videosignal to a selected pixel.

FIG. 27B illustrates another example of a block diagram of an activematrix display device, which is one example of the display device. Thedisplay device illustrated in FIG. 27B includes, over a substrate 5400,a pixel portion 5401 including a plurality of pixels each provided witha display element, a first scan line driver circuit 5402 and a secondscan line driver circuit 5404 for selecting a pixel, and a signal linedriver circuit 5403 for controlling input of a video signal to aselected pixel.

When a video signal input to a pixel of the display device in FIG. 27Bis a digital signal, the luminance of a pixel is controlled by switchingon/off of a transistor. In that case, display can be performed by anarea ratio grayscale method or a time ratio grayscale method, forexample. The area ratio grayscale method refers to a driving method bywhich one pixel is divided into a plurality of subpixels and thesubpixels are driven independently so that gray levels are expressed.The time ratio grayscale method refers to a driving method by which aperiod during which a transistor is on (or off) is controlled bydividing one frame period into a plurality of sub-frames, for example,so that gray levels are expressed. Since the response time oflight-emitting elements is shorter than that of liquid crystal elementsor the like, the light-emitting elements are suitable for the time ratiograyscale method.

As an example, in the display device illustrated in FIG. 27B, twoswitching TFTs are provided in one pixel; a signal input to a first scanline which is a gate wiring of one of the switching TFTs is generated inthe first scan line driver circuit 5402; and a signal input to a secondscan line which is a gate wiring of the other switching TFT is generatedin the second scan line driver circuit 5404. Note that one embodiment ofthe disclosed invention is not limited to this structure, and a signalinput to the first scan line and a signal input to the second scan linemay be generated by one scan line driver circuit. Further, the number ofscan lines used for controlling operation of switching elements issometimes increased depending on the number of switching TFTs includedin one pixel; in that case also, signals input to a plurality of scanlines may be generated by one scan line driver circuit or by a pluralityof scan line driver circuits.

The thin film transistors arranged in the pixel portion of the displaydevice can be formed according to the foregoing Embodiments. Moreover,some or all of the thin film transistors used in the driver circuit canbe formed over the substrate where the thin film transistors in thepixel portion are formed.

Note that a transistor with a light-transmitting property is notnecessarily formed in a peripheral driver circuit portion such as aprotective circuit, a gate driver, and a source driver. Accordingly, astructure may be employed in which light is transmitted through thepixel portion and light is not transmitted through the peripheral drivercircuit portion.

FIGS. 28A and 28B each illustrate the thin film transistors. FIG. 28Aillustrates the case where the thin film transistors are formed withoutusing a multi-tone mask. FIG. 28B illustrates the case where the thinfilm transistors are formed using a multi-tone mask. In the drawing, theleft portion indicates the transistor in the driver circuit portion, andthe right portion indicates the transistor in the pixel portion.

In the case where the thin film transistor in the driver circuit portionis formed without using a multi-tone mask, a conductive layer 2800functioning as a gate electrode is formed when the conductive layer 132a functioning as a gate wiring is formed, and conductive layers 2802 aand 2802 b functioning as a source electrode and a drain electrode areformed when the conductive layer 112 functioning as a source wiring isformed (see FIG. 28A, FIGS. 1A and 1B, and the like). In that case, itis not necessary to provide layers corresponding to the conductive layer126 a functioning as a gate electrode, the conductive layer 106 afunctioning as a source electrode, and the conductive layer 106 bfunctioning as a drain electrode of the transistor in the pixel portion;however, one embodiment of the disclosed invention is not limited tothis structure. Note that the source wiring and the source electrode(the drain wiring and the drain electrode) may be formed as a singlepart. In this specification, the distinction between a wiring and anelectrode is made only for convenience; therefore, if possible in termsof the structure, the wiring and the electrode may be formed as a singlepart or may be separately formed.

When the thin film transistor is formed using a multi-tone mask, awiring or an electrode has a layered structure of a conductive layerformed using a light-transmitting material and a conductive layer formedusing a low resistance material. For example, the gate electrode has alayered structure of a conductive layer 2810 formed using alight-transmitting material and a conductive layer 2812 formed using alow resistance material (see FIG. 28B). Moreover, the source electrodeor the drain electrode has a layered structure of a conductive layer2814 a (or a conductive layer 2814 b) formed using a light-transmittingmaterial and a conductive layer 2816 a (or a conductive layer 2816 b)formed using a low resistance material (see FIG. 28B). Note that since alow resistance material often has a light-shielding property, a thinfilm transistor to be formed does not transmit light, but does not needto have a complete light-shielding property (e.g., the lighttransmittance may be 10% or less).

The thin film transistor with a structure where light is not transmittedthrough the peripheral circuit portion is formed in such a manner,whereby resistance due to an electrode or the like can be reduced andcharacteristics of the thin film transistor can be improved.Accordingly, a semiconductor device in which the aperture ratio of apixel portion is improved and performance of a peripheral circuit isimproved can be provided. In other words, the object of improving thecharacteristics of the semiconductor device can be achieved.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 8

The semiconductor device can be applied as electronic paper. Electronicpaper can be used for electronic devices of a variety of fields fordisplaying data. For example, electronic paper can be used forelectronic book readers (e-book readers), posters, advertisement invehicles such as trains, display portions in a variety of cards such ascredit cards, and the like. Examples of the electronic devices areillustrated in FIGS. 29A and 29B and FIG. 30.

FIG. 29A illustrates a poster 2631 formed using electronic paper. Whenan advertising medium is printed paper, the advertisement is replaced byhands; when electronic paper is used, the advertising display can bechanged in a short time. Moreover, stable images can be obtained withoutdisplay deterioration. Note that the poster may have a structure capableof wirelessly transmitting and receiving data.

FIG. 29B illustrates an advertisement 2632 in a vehicle such as a train.When an advertising medium is printed paper, the advertisement isreplaced by hands; when electronic paper is used, the advertisingdisplay can be changed in a short time without much manpower. Further,stable images can be obtained without display deterioration. Note thatthe poster may have a structure capable of wirelessly transmitting andreceiving data.

FIG. 30 illustrates an example of an e-book reader 2700. For example,the e-book reader 2700 includes two housings: a housing 2701 and ahousing 2703. The housing 2701 and the housing 2703 are connected with ahinge 2711 so that the e-book reader 2700 can be opened and closed withthe hinge 2711 as an axis. With such a structure, the e-book reader 2700can operate like a paper book.

A display portion 2705 is incorporated in the housing 2701, and adisplay portion 2707 is incorporated in the housing 2703. The displayportions 2705 and 2707 may display one image or different images. Whenthe display portions 2705 and 2707 display different images, forexample, a display portion on the right side (the display portion 2705in FIG. 30) can display text and a display portion on the left side (thedisplay portion 2707 in FIG. 30) can display graphics.

FIG. 30 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, or the like may be provided onthe same surface as the display portion of the housing. Further, anexternal connection terminal (e.g., an earphone terminal, a USBterminal, or a terminal that can be connected to various cables such asan AC adapter and a USB cable), a recording medium insertion portion, orthe like may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may have a structure capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 9

In this embodiment, a structure and operation of a pixel which can beapplied to a liquid crystal display device will be described. As anoperation mode of a liquid crystal element in this embodiment, a TN(twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringefield switching) mode, an MVA (multi-domain vertical alignment) mode, aPVA (patterned vertical alignment) mode, an ASM (axially symmetricaligned micro-cell) mode, an OCB (optically compensated birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(antiferroelectric liquid crystal) mode, or the like can be used.

FIG. 40A shows an example of a pixel structure which can be applied to aliquid crystal display device. A pixel 5080 includes a transistor 5081,a liquid crystal element 5082, and a capacitor 5083. A gate of thetransistor 5081 is electrically connected to a wiring 5085. A firstterminal of the transistor 5081 is electrically connected to a wiring5084. A second terminal of the transistor 5081 is electrically connectedto a first terminal of the liquid crystal element 5082. A secondterminal of the liquid crystal element 5082 is electrically connected toa wiring 5087. A first terminal of the capacitor 5083 is electricallyconnected to the first terminal of the liquid crystal element 5082. Asecond terminal of the capacitor 5083 is electrically connected to awiring 5086. Note that a first terminal of a transistor refers to one ofa source and a drain, and a second terminal of the transistor refers tothe other of the source and the drain. That is, when the first terminalof the transistor is the source, the second terminal of the transistoris the drain. Similarly, when the first terminal of the transistor isthe drain, the second terminal of the transistor is the source.

The wiring 5084 can function as a signal line. The signal line is awiring for transmitting a signal voltage, which is input from theoutside of the pixel, to the pixel 5080. The wiring 5085 can function asa scan line. The scan line is a wiring for controlling on and off of thetransistor 5081. The wiring 5086 can function as a capacitor line. Thecapacitor line is a wiring for applying a predetermined voltage to thesecond terminal of the capacitor 5083. The transistor 5081 can functionas a switch. The capacitor 5083 can function as a storage capacitor. Thestorage capacitor is a capacitor with which the signal voltage continuesto be applied to the liquid crystal element 5082 even when the switch isoff. The wiring 5087 can function as a counter electrode. The counterelectrode is a wiring for applying a predetermined voltage to the secondterminal of the liquid crystal element 5082. Note that a function ofeach wiring is not limited thereto, and each wiring can have a varietyof functions. For example, by changing a voltage applied to thecapacitor line, a voltage applied to the liquid crystal element can beadjusted. Note that the transistor 5081 can be a p-channel transistor oran n-channel transistor because the transistor 5081 merely functions asa switch.

FIG. 40B illustrates an example of a pixel structure which can beapplied to a liquid crystal display device. The example of the pixelstructure illustrated in FIG. 40B is the same as that in FIG. 40A,except that the wiring 5087 is eliminated and the second terminal of theliquid crystal element 5082 and the second terminal of the capacitor5083 are electrically connected to each other. The example of the pixelstructure in FIG. 40B can be particularly applied to the case of using aliquid crystal element with a horizontal electric field mode (includingan IPS mode and FFS mode). This is because in the horizontal electricfield mode liquid crystal element, the second terminal of the liquidcrystal element 5082 and the second terminal of the capacitor 5083 canbe formed over one substrate; thus, it is easy to electrically connectthe second terminal of the liquid crystal element 5082 and the secondterminal of the capacitor 5083. With the pixel structure illustrated inFIG. 40B, the wiring 5087 can be eliminated, so that a manufacturingprocess can be simplified and manufacturing costs can be reduced.

A plurality of pixel structures illustrated in FIG. 40A or FIG. 40B canbe arranged in matrix. Accordingly, a display portion of a liquidcrystal display device is formed, so that a variety of images can bedisplayed. FIG. 40C illustrates a circuit configuration in the casewhere a plurality of pixel structures illustrated in FIG. 40A arearranged in matrix. FIG. 40C is a circuit diagram illustrating fourpixels among a plurality of pixels included in the display portion. Apixel arranged in an i-th column and a j-th row (each of i and j is anatural number) is represented as a pixel 5080 _(—) i,j, and a wiring5084 _(—) i, a wiring 5085 _(—) j, and a wiring 5086 _(—) j areelectrically connected to the pixel 5080 _(—) i,j. Similarly, a pixel5080 _(—) i+1,j is electrically connected to a wiring 5084 _(—) i+1, thewiring 5085 _(—) j, and the wiring 5086 _(—) j. Similarly, a pixel 5080_(—) i,j+1 is electrically connected to the wiring 5084 _(—) i, a wiring5085 _(—) j+1, and a wiring 5086 _(—) j+1. Similarly, a pixel 5080 _(—)i+1,j+1 is electrically connected to the wiring 5084 _(—) i+1, thewiring 5085 _(—) j+1, and the wiring 5086 _(—) j+1. Note that eachwiring can be used in common with a plurality of pixels in the samecolumn or the same row. In the pixel structure illustrated in FIG. 40C,the wiring 5087 is a counter electrode, which is used by all the pixelsin common; therefore, the wiring 5087 is not indicated by the naturalnumber i or j. Further, since the pixel structure in FIG. 40B can alsobe used, the wiring 5087 is not essential even in a structure where thewiring 5087 is described, and the wiring 5087 can be eliminated whenanother wiring functions as the wiring 5087, for example.

The pixel structure in FIG. 40C can be driven by a variety of drivingmethods. In particular, when the pixels are driven by a method calledalternating-current driving, degradation (burn-in) of the liquid crystalelement can be suppressed. FIG. 40D is a timing chart of voltagesapplied to each wiring in the pixel structure in FIG. 40C in the casewhere dot inversion driving which is a kind of alternating-currentdriving is performed. By the dot inversion driving, flickers seen whenthe alternating-current driving is performed can be suppressed.

In the pixel structure in FIG. 40C, a switch in a pixel electricallyconnected to the wiring 5085 _(—) j is brought into a selection state(an on state) in a j-th gate selection period in one frame period, andbrought into a non-selection state (an off state) in the other periods.Then, after the j-th gate selection period, a (j+1)th gate selectionperiod is provided. By performing sequential scanning in this manner,all the pixels are sequentially selected in one frame period. In thetiming chart in FIG. 40D, when the voltage is set to high level, theswitch in the pixel is brought into a selection state; when the voltageis set to low level, the switch is brought into a non-selection state.Note that this is the case where the transistors in the pixels aren-channel transistors. In the case of using p-channel transistors, therelation between the voltage and the selection state is opposite to thatin the case of using n-channel transistors.

In the timing chart illustrated in FIG. 40D, in the j-th gate selectionperiod in a k-th frame (k is a natural number), a positive signalvoltage is applied to the wiring 5084 _(—) i used as a signal line, anda negative signal voltage is applied to the wiring 5084 _(—) i+1. Then,in the (j+1)th gate selection period in the k-th frame, a negativesignal voltage is applied to the wiring 5084 _(—) i, and a positivesignal voltage is applied to the wiring 5084 _(—) i+1. After that,signals whose polarity is reversed in each gate selection period arealternately supplied to each of the signal lines. Thus, in the k-thframe, the positive signal voltage is applied to the pixel 5080 _(—) i,jand the pixel 5080 _(—) i+1,j+1, and the negative signal voltage isapplied to the pixel 5080 _(—) i+1,j and the pixel 5080 _(—) i,j+1.Then, in a (k+1)th frame, a signal voltage whose polarity is opposite tothat of the signal voltage written in the k-th frame is written to eachpixel. Thus, in the (k+1)th frame, the positive signal voltage isapplied to the pixel 5080 _(—) i+1,j and the pixel 5080 _(—) i,j+1, andthe negative signal voltage is applied to the pixel 5080 _(—) i,j andthe pixel 5080 _(—) i+1,j+1. The dot inversion driving is a drivingmethod in which signal voltages whose polarity is different betweenadjacent pixels are applied in one frame and the polarity of the voltagesignal for one pixel is reversed in each frame as described above. Bythe dot inversion driving, flickers seen when the entire or part of animage to be displayed is uniform can be suppressed while degradation ofthe liquid crystal element is suppressed. Note that voltages applied toall the wirings 5086 including the wiring 5086 _(—) j and the wiring5086 _(—) j+1 can be a constant voltage. Moreover, although only thepolarity of the signal voltages for the wirings 5084 is shown in thetiming chart, the signal voltages can actually have a variety of valuesin the polarity shown. Here, the case where the polarity is reversed perdot (per pixel) is described; however, this embodiment is not limitedthereto, and the polarity can be reversed per a plurality of pixels. Forexample, when the polarity of signal voltages to be written is reversedper two gate selection periods, power consumed by writing the signalvoltages can be reduced. Alternatively, the polarity may be reversed percolumn (source line inversion) or per row (gate line inversion).

Note that a constant voltage may be applied to the second terminal ofthe capacitor 5083 in the pixel 5080 in one frame period. Here, avoltage applied to the wiring 5085 used as a scan line is at low levelin most of one frame period, which means that a substantially constantvoltage is applied to the wiring 5085; therefore, the second terminal ofthe capacitor 5083 in the pixel 5080 may be connected to the wiring5085. FIG. 40E illustrates an example of a pixel structure which can beapplied to a liquid crystal display device. Compared to the pixelstructure in FIG. 40C, features of the pixel structure in FIG. 40E arethat the wiring 5086 is eliminated and the second terminal of thecapacitor 5083 in the pixel 5080 and the wiring 5085 in the previous roware electrically connected to each other. Specifically, in the rangeillustrated in FIG. 40E, the second terminals of the capacitors 5083 inthe pixel 5080 _(—) i,j+1 and the pixel 5080 _(—) i+1,j+1 areelectrically connected to the wiring 5085 _(—) j. The wiring 5086 can beeliminated when the second terminals of the capacitors 5083 in thepixels 5080 and the wiring 5085 in the previous row are electricallyconnected to each other in such a manner, so that the aperture ratio ofthe pixel can be increased. Note that the second terminal of thecapacitor 5083 may be connected to the wiring 5085 in another rowinstead of the wiring 5085 in the previous row. Further, the pixelstructure in FIG. 40E can be driven by a driving method similar to thatin the pixel structure in FIG. 40C.

Note that a voltage applied to the wiring 5084 used as a signal line canbe made lower by using the capacitor 5083 and the wiring electricallyconnected to the second terminal of the capacitor 5083. A structure anda driving method of a pixel in that case are described with reference toFIGS. 40F and 40G. Compared to the pixel structure in FIG. 40A, featuresof the pixel structure in FIG. 40F are that two wirings 5086 areprovided per pixel row, and in adjacent pixels, one wiring iselectrically connected to every other second terminal of the capacitors5083 and the other wiring is electrically connected to the remainingevery other second terminal of the capacitors 5083. Two wirings 5086 arereferred to as a wiring 5086-1 and a wiring 5086-2. Specifically, in therange illustrated in FIG. 40F, the second terminal of the capacitor 5083in the pixel 5080 _(—) i,j is electrically connected to a wiring 5086-1_(—) j; the second terminal of the capacitor 5083 in the pixel 5080 _(—)i+1,j is electrically connected to a wiring 5086-2 _(—) j; the secondterminal of the capacitor 5083 in the pixel 5080 _(—) i,j+1 iselectrically connected to a wiring 5086-2 _(—) j+1; and the secondterminal of the capacitor 5083 in the pixel 5080 _(—) i+1,j+1 iselectrically connected to a wiring 5086-1 _(—) j+1.

For example, as illustrated in FIG. 40G, when a positive signal voltageis written to the pixel 5080 _(—) i,j in the k-th frame, the wiring5086-1 _(—) j is set to low level in the j-th gate selection period andis changed to high level after the j-th gate selection period. Then, thewiring 5086-1 _(—) j is kept at high level in one frame period, andafter a negative signal voltage is written in the j-th gate selectionperiod in the (k+1)th frame, the wiring 5086-1 _(—) j is changed to lowlevel. In such a manner, a voltage of the wiring which is electricallyconnected to the second terminal of the capacitor 5083 is changed in thepositive direction after a positive signal voltage is written to thepixel, whereby a voltage applied to the liquid crystal element can bechanged in the positive direction by a predetermined amount. That is, asignal voltage written to the pixel can be reduced accordingly, so thatpower consumed by signal writing can be reduced. Note that when anegative signal voltage is written in the j-th gate selection period, avoltage of the wiring which is electrically connected to the secondterminal of the capacitor 5083 is changed in the negative directionafter a negative signal voltage is written to the pixel. Accordingly, avoltage applied to the liquid crystal element can be changed in thenegative direction by a predetermined amount, so that the signal voltagewritten to the pixel can be reduced as in the case of the positivepolarity. In other words, as for the wiring which is electricallyconnected to the second terminal of the capacitor 5083, differentwirings are preferably used for a pixel to which a positive signalvoltage is applied and a pixel to which a negative signal voltage isapplied in the same row in one frame. FIG. 40F illustrates the examplein which the wiring 5086-1 is electrically connected to the pixels towhich a positive signal voltage is applied in the k-th frame, and thewiring 5086-2 is electrically connected to the pixels to which anegative signal voltage is applied in the k-th frame. Note that this isjust an example, and for example, in the case of using a driving methodin which pixels to which a positive signal voltage is applied and pixelsto which a negative signal voltage is applied are arranged every twopixels, the wirings 5086-1 and 5086-2 are preferably electricallyconnected to every alternate two pixels accordingly. Furthermore, in thecase where signal voltages of the same polarity are written in all thepixels in one row (gate line inversion), one wiring 5086 is provided perrow. In other words, the pixel structure in FIG. 40C can employ thedriving method where a signal voltage written to a pixel is reduced asdescribed with reference to FIGS. 40F and 40G.

Next, a pixel structure and a driving method are described which arepreferably used particularly by a liquid crystal element with a verticalalignment (VA) mode typified by an MVA mode or a PVA mode. The VA modehas advantages that a rubbing process is not necessary in manufacturing,the amount of light leakage is small in displaying black images, and thelevel of drive voltage is low; however, the VA mode has a problem inthat the quality of images deteriorates when a screen is viewed from anangle (the viewing angle is small). In order to increase the viewingangle in the VA mode, a pixel structure where one pixel includes aplurality of subpixels as illustrated in FIGS. 41A and 41B is effective.Pixel structures illustrated in FIGS. 41A and 41B are examples of thecase where the pixel 5080 includes two subpixels (a subpixel 5080-1 anda subpixel 5080-2). Note that the number of subpixels in one pixel isnot limited to two and can be other numbers. As the number of subpixelsbecomes larger, the viewing angle can be further increased. A pluralityof subpixels can have the same circuit configuration. Here, the case isdescribed in which all the subpixels have the same circuit configurationas that in FIG. 40A. The first subpixel 5080-1 includes a transistor5081-1, a liquid crystal element 5082-1, and a capacitor 5083-1. Theconnection relation is the same as that in the circuit configuration inFIG. 40A. Similarly, the second subpixel 5080-2 includes a transistor5081-2, a liquid crystal element 5082-2, and a capacitor 5083-2. Theconnection relation is the same as that in the circuit configuration inFIG. 40A.

The pixel structure in FIG. 41A includes, for two subpixels included inone pixel, two wirings 5085 (a wiring 5085-1 and a wiring 5085-2) usedas scan lines, one wiring 5084 used as a signal line, and one wiring5086 used as a capacitor line. When the signal line and the capacitorline are shared with two subpixels in such a manner, the aperture ratiocan be increased. Further, a signal line driver circuit can besimplified, so that manufacturing costs can be reduced. Moreover, thenumber of connections between a liquid crystal panel and a drivercircuit IC can be reduced, so that the yield can be increased. The pixelstructure in FIG. 41B includes, for two subpixels included in one pixel,one wiring 5085 used as a scan line, two wirings 5084 (a wiring 5084-1and a wiring 5084-2) used as signal lines, and one wiring 5086 used as acapacitor line. When the scan line and the capacitor line are sharedwith two subpixels in such a manner, the aperture ratio can beincreased. Further, the total number of scan lines can be reduced, sothat one gate line selection period can be sufficiently long even in ahigh-definition liquid crystal panel, and an appropriate signal voltagecan be written in each pixel.

FIGS. 41C and 41D each schematically illustrate an example of electricalconnections of elements in the case where the liquid crystal element inthe pixel structure in FIG. 41B is replaced with the shape of a pixelelectrode. In FIGS. 41C and 41D, an electrode 5088-1 represents a firstpixel electrode, and an electrode 5088-2 represents a second pixelelectrode. In FIG. 41C, the first pixel electrode 5088-1 corresponds toa first terminal of the liquid crystal element 5082-1 in FIG. 41B, andthe second pixel electrode 5088-2 corresponds to a first terminal of theliquid crystal element 5082-2 in FIG. 41B. That is, the first pixelelectrode 5088-1 is electrically connected to one of a source and adrain of the transistor 5081-1, and the second pixel electrode 5088-2 iselectrically connected to one of a source and a drain of the transistor5081-2. In FIG. 41D, the connection relation between the pixel electrodeand the transistor is opposite to that in FIG. 41C. That is, the firstpixel electrode 5088-1 is electrically connected to one of the sourceand the drain of the transistor 5081-2, and the second pixel electrode5088-2 is electrically connected to one of the source and the drain ofthe transistor 5081-1.

By alternately arranging a plurality of pixel structures illustrated inFIGS. 41C and 41D in matrix, special advantageous effects can beobtained. FIGS. 41E and 41F illustrate an example of such a pixelstructure and driving method. In the pixel structure in FIG. 41E,portions corresponding to the pixel 5080 _(—) i,j and the pixel 5080_(—) j+1, j+1 have the structure illustrated in FIG. 41C, and portionscorresponding to the pixel 5080 _(—) j+1,j and the pixel 5080 _(—) i,j+1have the structure illustrated in FIG. 41D. When the pixels with thisstructure are driven as the timing chart illustrated in FIG. 41F, in thej-th gate selection period in the k-th frame, a positive signal voltageis written to the first pixel electrode in the pixel 5080 _(—) i,j andthe second pixel electrode in the pixel 5080 _(—) i+1,j, and a negativesignal voltage is written to the second pixel electrode in the pixel5080 _(—) i,j and the first pixel electrode in the pixel 5080 _(—)i+1,j. Then, in the (j+1)th gate selection period in the k-th frame, apositive signal voltage is written to the second pixel electrode in thepixel 5080 _(—) i,j+1 and the first pixel electrode in the pixel 5080_(—) j+1,j+1, and a negative signal voltage is written to the firstpixel electrode in the pixel 5080 _(—) i,j+1 and the second pixelelectrode in the pixel 5080 _(—) i+1,j+1. In the (k+1)th frame, thepolarity of the signal voltage is reversed in each pixel. Accordingly,the polarity of the voltage applied to the signal line can be the samein one frame period while driving corresponding to dot inversion drivingis realized in the pixel structure including subpixels, whereby powerconsumed by writing the signal voltages to the pixels can be drasticallyreduced. Note that voltages applied to all the wirings 5086 includingthe wiring 5086 _(—) j and the wiring 5086 _(—) j+1 can be a constantvoltage.

Further, with a pixel structure and a driving method illustrated inFIGS. 41G and 41H, the level of the signal voltage written to a pixelcan be reduced. In the structure, capacitor lines which are electricallyconnected to a plurality of subpixels included in each pixel aredifferent between the subpixels. That is, with the pixel structure andthe driving method illustrated in FIGS. 41G and 41H, one capacitor lineis shared with subpixels in one row, to which signal voltages of thesame polarity are written in one frame; and subpixels to which signalvoltages of the different polarities are written in one frame usedifferent capacitor lines in one row. Then, when writing in each row isfinished, voltages of the capacitor lines are changed in the positivedirection in the subpixels to which a positive signal voltage iswritten, and changed in the negative direction in the subpixels to whicha negative signal voltage is written; thus, the level of the signalvoltage written to the pixel can be reduced. Specifically, two wirings5086 (the wirings 5086-1 and 5086-2) used as capacitor lines areprovided per row. The first pixel electrode in the pixel 5080 _(—) i,jand the wiring 5086-1 _(—) j are electrically connected through thecapacitor. The second pixel electrode in the pixel 5080 _(—) i,j and thewiring 5086-2 _(—) j are electrically connected through the capacitor.The first pixel electrode in the pixel 5080 _(—) i+1,j and the wiring5086-2 _(—) j are electrically connected through the capacitor. Thesecond pixel electrode in the pixel 5080 _(—) i+1,j and the wiring5086-1 _(—) j are electrically connected through the capacitor. Thefirst pixel electrode in the pixel 5080 _(—) i,j+1 and the wiring 5086-2_(—) j+1 are electrically connected through the capacitor. The secondpixel electrode in the pixel 5080 _(—) i,j+1 and the wiring 5086-1 _(—)j+1 are electrically connected through the capacitor. The first pixelelectrode in the pixel 5080 _(—) i+1, j+1 and the wiring 5086-1 _(—) j+1are electrically connected through the capacitor. The second pixelelectrode in the pixel 5080 _(—) i+1,j+1 and the wiring 5086-2 _(—) j+1are electrically connected through the capacitor. Note that this is justan example, and for example, in the case of using a driving method inwhich pixels to which a positive signal voltage is applied and pixels towhich a negative signal voltage is applied are arranged every twopixels, the wirings 5086-1 and 5086-2 are preferably electricallyconnected to every alternate two pixels accordingly. Furthermore, in thecase where signal voltages of the same polarity are written in all thepixels in one row (gate line inversion), one wiring 5086 is provided perrow. In other words, the pixel structure in FIG. 41E can employ thedriving method where a signal voltage written to a pixel is reduced asdescribed with reference to FIGS. 41G and 41H.

Embodiment 10

Next, another structure example and a driving method of a display devicewill be described. In this embodiment, a display device including adisplay element whose luminance response with respect to signal writingis slow (whose response time is long) will be described. In thisembodiment, a liquid crystal element is described as an example of thedisplay element with long response time; however, a display element inthis embodiment is not limited the liquid crystal element, and a varietyof display elements whose luminance response with respect to signalwriting is slow can be used.

In a general liquid crystal display device, luminance response withrespect to signal writing is slow, and it sometimes takes more than oneframe period to complete the response even when a signal voltagecontinues to be applied to a liquid crystal element. Moving imagescannot be precisely displayed by such a display element. Further, in thecase of employing active matrix driving, the time for signal writing toone liquid crystal element is usually only a period (one scan lineselection period) obtained by dividing a signal writing cycle (one frameperiod or one subframe period) by the number of scan lines, and theliquid crystal element cannot respond in such a short time in manycases. Accordingly, most of the response of the liquid crystal elementis performed in a period during which signal writing is not performed.Here, the dielectric constant of the liquid crystal element is changedin accordance with the transmittance of the liquid crystal element, andthe response of the liquid crystal element in a period during whichsignal writing is not performed means that the dielectric constant ofthe liquid crystal element is changed when electric charge is notexchanged with the outside of the liquid crystal element (in a constantcharge state). In other words, in the formula wherecharge=(capacitance)·(voltage), the capacitance is changed when thecharge is constant. Accordingly, a voltage applied to the liquid crystalelement is changed from a voltage at the time of signal writing, inaccordance with the response of the liquid crystal element. Therefore,when the liquid crystal element whose luminance response with respect tosignal writing is slow is driven by an active matrix mode, a voltageapplied to the liquid crystal element cannot theoretically reach thevoltage at the time of signal writing.

In a display device in this embodiment, the signal level at the time ofsignal writing is corrected in advance (a correction signal is used) sothat a display element can reach desired luminance within a signalwriting cycle, whereby the above problem can be solved. Further, sincethe response time of the liquid crystal element is shorter as the signallevel becomes higher, the response time of the liquid crystal elementcan also be reduced by writing a correction signal. A driving method bywhich such a correction signal is added is referred to as overdriving.By overdriving in this embodiment, even when a signal writing cycle isshorter than a cycle (an input image signal cycle T_(in)) for an imagesignal input to the display device, the signal level is corrected inaccordance with the signal writing cycle, whereby the display elementcan reach desired luminance within the signal writing cycle. An exampleof the case where the signal writing cycle is shorter than the inputimage signal cycle T_(in) is the case where one original image isdivided into a plurality of subimages and the plurality of subimages aresequentially displayed in one frame period.

Next, an example of a method for correcting the signal level at the timeof signal writing in an active matrix display device is described withreference to FIGS. 42A and 42B. FIG. 42A is a graph schematicallyillustrating change over time in signal level at the time of signalwriting in one display element, with the time as the horizontal axis andthe signal level at the time of signal writing as the vertical axis.FIG. 42B is a graph schematically illustrating change over time indisplay level in one display element, with the time as the horizontalaxis and the display level as the vertical axis. Note that when thedisplay element is a liquid crystal element, the signal level at thetime of signal writing can be the voltage, and the display level can bethe transmittance of the liquid crystal element. In the followingdescription, the vertical axis in FIG. 42A represents voltage, and thevertical axis in FIG. 42B represents transmittance. Note that in theoverdriving in this embodiment, the signal level may be other thanvoltage (may be the duty ratio or current, for example). Moreover, inthe overdriving in this embodiment, the display level may be other thantransmittance (may be luminance or current, for example). Liquid crystalelements are classified into two modes: a normally black mode in whichblack is displayed when a voltage is 0 (e.g., a VA mode and an IPSmode), and a normally white mode in which white is displayed when avoltage is 0 (e.g., a TN mode and an OCB mode). The graph in FIG. 42Bcan correspond to both modes; the transmittance increases in the upperpart of the graph in the normally black mode, whereas the transmittanceincreases in the lower part of the graph in the normally white mode.That is, a liquid crystal mode in this embodiment may be a normallyblack mode or a normally white mode. Note that the timing of signalwriting is represented on the time axis by dotted lines, and a periodafter signal writing is performed until the next signal writing isperformed is referred to as a retention period F_(i). In thisembodiment, i is an integer and an index for representing each retentionperiod. In FIGS. 42A and 42B, i is 0 to 2; however, i can be an integerother than 0 to 2 (only the case where i is 0 to 2 is illustrated). Notethat in the retention period F_(i), the transmittance for realizingluminance corresponding to an image signal is denoted by T_(i), and thevoltage for providing the transmittance T_(i) in a constant state isdenoted by V_(i). In FIG. 42A, a dashed line 5101 represents change overtime in voltage applied to the liquid crystal element in the case whereoverdriving is not performed, and a solid line 5102 represents changeover time in voltage applied to the liquid crystal element in the casewhere the overdriving in this embodiment is performed. Similarly, inFIG. 42B, a dashed line 5103 represents change over time intransmittance of the liquid crystal element when overdriving is notperformed, and a solid line 5104 represents change over time intransmittance of the liquid crystal element when the overdriving in thisembodiment is performed. Note that the difference between the desiredtransmittance T_(i) and the actual transmittance at the end of theretention period F_(i) is referred to as an error α_(i).

It is assumed that, in the graph illustrated in FIG. 42A, both thedashed line 5101 and the solid line 5102 represent the case where adesired voltage V₀ is applied in a retention period F₀; and in the graphillustrated in FIG. 42B, both the dashed line 5103 and the solid line5104 represent the case where desired transmittance T₀ is obtained. Whenoverdriving is not performed, a desired voltage V₁ is applied at thebeginning of a retention period F₁ as shown by the dashed line 5101. Ashas been described above, a period for signal writing is extremelyshorter than a retention period, and the liquid crystal element is in aconstant charge state in most of the retention period. Accordingly, avoltage applied to the liquid crystal element in the retention period ischanged along with the change in transmittance and becomes greatlydifferent from the desired voltage V₁ at the end of the retention periodF₁. In this case, the dashed line 5103 in the graph of FIG. 42B is alsogreatly different from desired transmittance T₁. Thus, accurate displayof an image signal cannot be performed, and the image quality isdegraded. On the other hand, when the overdriving in this embodiment isperformed, a voltage V₁′ which is higher than the desired voltage V₁ isapplied to the liquid crystal element at the beginning of the retentionperiod F₁ as shown by the solid line 5102. That is, the voltage V₁′which is corrected from the desired voltage V₁ is applied to the liquidcrystal element at the beginning of the retention period F₁ so that thevoltage applied to the liquid crystal element at the end of theretention period F₁ is close to the desired voltage V₁ in anticipationof gradual change in voltage applied to the liquid crystal element inthe retention period F₁. Accordingly, the desired voltage V₁ can beaccurately applied to the liquid crystal element. At that time, as shownby the solid line 5104 in the graph of FIG. 42B, the desiredtransmittance T₁ can be obtained at the end of the retention period F₁.In other words, the response of the liquid crystal element within thesignal writing cycle can be realized, despite the fact that the liquidcrystal element is in a constant charge state in most of the retentionperiod. Then, in a retention period F₂, the case where a desired voltageV₂ is lower than V₁ is shown. In that case also, as in the retentionperiod F₁, a voltage V₂′ which is corrected from the desired voltage V₂may be applied to the liquid crystal element at the beginning of theretention period F₂ so that the voltage applied to the liquid crystalelement at the end of the retention period F₂ is close to the desiredvoltage V₂ in anticipation of gradual change in voltage applied to theliquid crystal element in the retention period F₂. Accordingly, as shownby the solid line 5104 in the graph of FIG. 42B, desired transmittanceT₂ can be obtained at the end of the retention period F₂. Note that whenV_(i) is higher than V_(i−1) as in the retention period F₁, thecorrected voltage V_(i)′ is preferably corrected to be higher than adesired voltage V_(i). Further, when V_(i) is lower than V_(i−1) as inthe retention period F₂, the corrected voltage V_(i)′ is preferablycorrected to be lower than the desired voltage V_(i). A specificcorrection value can be derived by measuring response characteristics ofthe liquid crystal element in advance. As a method for realizing theoverdriving in the device, a method in which a correction formula isformulated and included in a logic circuit, a method in which acorrection value is stored in a memory as a lookup table and read asnecessary, or the like can be used.

Note that there are several limitations on the actual realization of theoverdriving in this embodiment as a device. For example, voltagecorrection should be performed in the range of the rated voltage of asource driver. That is, if a desired voltage is originally high and anideal correction voltage exceeds the rated voltage of the source driver,complete correction cannot be performed. Problems in such a case aredescribed with reference to FIGS. 42C and 42D. As in FIG. 42A, FIG. 42Cis a graph in which change over time in voltage in one liquid crystalelement is schematically illustrated as a solid line 5105 with the timeas the horizontal axis and the voltage as the vertical axis. As in FIG.42B, FIG. 42D is a graph in which change over time in transmittance ofone liquid crystal element is schematically illustrated as a solid line5106 with the time as the horizontal axis and the transmittance as thevertical axis. Note that other references are similar to those in FIGS.42A and 42B; therefore, the description is not repeated. FIGS. 42C and42D illustrate a state where sufficient correction is not performedbecause the correction voltage V₁′ for realizing the desiredtransmittance T₁ in the retention period F₁ exceeds the rated voltage ofthe source driver, and thus V₁′=V₁ has to be given. At that time, thetransmittance at the end of the retention period F₁ is deviated from thedesired transmittance T₁ by the error α₁. Note that the error α₁ isincreased only when the desired voltage is originally high; therefore,degradation of image quality due to occurrence of the error α₁ is oftenin the allowable range. However, as the error α₁ is increased, an errorin the algorithm for voltage correction is also increased. In otherwords, in the algorithm for voltage correction, when it is assumed thatthe desired transmittance is obtained at the end of the retentionperiod, even though the error α₁ is increased, the voltage correction isperformed on the basis that the error α₁ is small. Accordingly, theerror is included in the correction in the next retention period F₂, andthus, an error α₂ is also increased. Moreover, when the error α₂ isincreased, the following error α₃ is further increased, and the error isincreased in a chain reaction manner, resulting in significantdegradation of image quality. In the overdriving in this embodiment, inorder to prevent increase of errors in such a chain reaction manner,when the correction voltage V_(i)′ exceeds the rated voltage of thesource driver in the retention period F_(i), an error α_(i) at the endof the retention period F_(i) is assumed, and the correction voltage ina retention period F_(i+1) can be adjusted in consideration of theamount of the error α_(i). Accordingly, even when the error α_(i) isincreased, the adverse effect of the error α_(i) on the error α_(i+1)can be minimized, whereby increase of errors in a chain reaction mannercan be prevented. An example where the error α₂ is minimized in theoverdriving in this embodiment is described with reference to FIGS. 42Eand 42F. In a graph of FIG. 42E, a solid line 5107 represents changeover time in voltage in the case where the correction voltage V₂′ in thegraph of FIG. 42C is further adjusted to be a correction voltage V₂″. Agraph of FIG. 42F illustrates change over time in transmittance in thecase where a voltage is corrected in accordance with the graph of FIG.42E. The solid line 5106 in the graph of FIG. 42D indicates thatexcessive correction (i.e., correction in a situation where an error islarge) is caused by the correction voltage V₂′. On the other hand, asolid line 5108 in the graph of FIG. 42F indicates that excessivecorrection is suppressed by the correction voltage V₂″, which isadjusted in consideration of the error α₁, and the error α₂ isminimized. A specific correction value can be derived by measuringresponse characteristics of the liquid crystal element in advance. As amethod for realizing the overdriving in the device, a method in which acorrection formula is formulated and included in a logic circuit, amethod in which a correction value is stored in a memory as a lookuptable and read as necessary, or the like can be used. Moreover, such amethod can be added separately from a portion for calculating acorrection voltage V_(i)′ or included in the portion for calculating thecorrection voltage V_(i)′. Note that the amount of correction of acorrection voltage V_(i)″ which is adjusted in consideration of an errorα_(i−1) (the difference with the desired voltage V_(i)) is preferablysmaller than that of V_(i)′. That is, |V_(i)″−V_(i)|<|V_(i)′−V_(i)| ispreferable.

Note that the error α_(i) which is caused because an ideal correctionvoltage exceeds the rated voltage of the source driver is increased as asignal writing cycle is shorter. This is because the response time ofthe liquid crystal element needs to be shorter as the signal writingcycle is shorter, and thus, the higher correction voltage is necessary.Further, as a result of increasing the correction voltage needed, thecorrection voltage exceeds the rated voltage of the source driver morefrequently, whereby large errors α_(i) occur more frequently.Accordingly, the overdriving in this embodiment is more effective in thecase where the signal writing cycle is shorter. Specifically, theoverdriving in this embodiment is significantly effective in the case ofperforming the following driving methods, for example: the case whereone original image is divided into a plurality of subimages and theplurality of subimages are sequentially displayed in one frame period,the case where motion of a plurality of images is detected and anintermediate image of the plurality of images is generated andinterpolated between the plurality of images (so-called motioncompensation frame rate conversion), and the case where such drivingmethods are combined.

Note that a rated voltage of the source driver has the lower limit inaddition to the upper limit described above. An example of the lowerlimit is the case where a voltage lower than the voltage 0 cannot beapplied. At that time, since an ideal correction voltage cannot beapplied as in the case of the upper limit described above, the errorα_(i) is increased. However, in that case also, the error α_(i) at theend of the retention period F_(i) is assumed, and the correction voltagein the retention period F_(i+1) can be adjusted in consideration of theamount of the error α_(i) in a similar manner as the above method. Notethat when a voltage (a negative voltage) lower than the voltage 0 can beapplied as a rated voltage of the source driver, the negative voltagemay be applied to the liquid crystal element as a correction voltage.Accordingly, the voltage applied to the liquid crystal element at theend of retention period F_(i) can be adjusted to be close to the desiredvoltage V_(i) in anticipation of change in potential due to a constantcharge state.

In addition, in order to suppress degradation of the liquid crystalelement, so-called inversion driving in which the polarity of a voltageapplied to the liquid crystal element is periodically reversed can beperformed in combination with the overdriving. That is, the overdrivingin this embodiment includes, in its category, the case where theoverdriving is performed at the same time as the inversion driving. Forexample, in the case where the length of the signal writing cycle is ½of that of the input image signal cycle T_(in), when the length of acycle for reversing the polarity is approximately the same as that ofthe input image signal cycle T_(in), two sets of writing of a positivesignal and two sets of writing of a negative signal are alternatelyperformed. The length of the cycle for reversing the polarity is madelarger than that of the signal writing cycle in such a manner, wherebythe frequency of charge and discharge of a pixel can be reduced, so thatpower consumption can be reduced. Note that when the cycle for reversingthe polarity is made too long, a defect sometimes occurs in whichluminance difference due to the difference of polarity is recognized asa flicker; therefore, it is preferable that the length of the cycle forreversing the polarity be substantially the same as or smaller than thatof the input image signal cycle T_(in).

Embodiment 11

Next, another structure example and a driving method of a display devicewill be described. In this embodiment, a method will be described bywhich an image that compensates motion of an image (an input image)which is input from the outside of a display device is generated insidethe display device on the basis of a plurality of input images and thegenerated image (the generation image) and the input image aresequentially displayed. Note that when an image for interpolating motionof an input image is a generation image, motion of moving images can bemade smooth, and decrease in quality of moving images because ofafterimages or the like due to hold driving can be suppressed. Here,moving image interpolation is described below. Ideally, display ofmoving images is realized by controlling the luminance of each pixel inreal time; however, individual control of pixels in real time hasproblems such as the enormous number of control circuits, space forwirings, and the enormous amount of input image data. Thus, it isdifficult to realize the individual control of pixels. Therefore, fordisplay of moving images by a display device, a plurality of stillimages are sequentially displayed in a certain cycle so that displayappears to be moving images. The cycle (in this embodiment, referred toas an input image signal cycle and denoted by T_(in)) is standardized,and for example, 1/60 second in NTSC and 1/50 second in PAL. Such acycle does not cause a problem of moving image display in a CRT, whichis an impulsive display device. However, in a hold-type display device,when moving images conforming to these standards are displayed withoutchange, a defect (hold blur) in which display is blurred because ofafterimages or the like due to hold driving occurs. Since hold blur isrecognized by discrepancy between unconscious motion interpolation dueto human eye tracking and hold-type display, the hold blur can bereduced by making the input image signal cycle shorter than that inconventional standards (by making the control closer to individualcontrol of pixels in real time). However, it is difficult to reduce thelength of the input image signal cycle because the standard needs to bechanged and the amount of data is increased. However, when an image forinterpolating motion of an input image is generated inside the displaydevice on the basis of a standardized input image signal and display isperformed while the generation image interpolates the input image, holdblur can be reduced without change in the standard or increase in theamount of data. Operation such that an image signal is generated insidethe display device on the basis of an input image signal to interpolatemotion of the input image is referred to as moving image interpolation.

By a method for interpolating moving images in this embodiment, motionblur can be reduced. The method for interpolating moving images in thisembodiment can include an image generation method and an image displaymethod. Further, by using a different image generation method and/or adifferent image display method for motion with a specific pattern,motion blur can be effectively reduced. FIGS. 43A and 43B are schematicdiagrams each illustrating an example of a method for interpolatingmoving images in this embodiment. FIGS. 43A and 43B each illustratetiming of treating each image by using the position of the horizontaldirection, with the time as the horizontal axis. A portion representedas “input” indicates timing at which an input image signal is input.Here, images 5121 and 5122 are focused as two images that are temporallyadjacent to each other. An input image is input at an interval of thecycle T_(in). Note that the length of one cycle T_(in) is referred to asone frame or one frame period in some cases. A portion represented as“generation” indicates timing at which a new image is generated from aninput image signal. Here, an image 5123 which is a generation imagegenerated on the basis of the images 5121 and 5122 is focused. A portionrepresented as “display” indicates timing at which an image is displayedin the display device. Note that images other than the focused imagesare only represented by dashed lines, and by treating such images in amanner similar to that of the focused images, the example of the methodfor interpolating moving images in this embodiment can be realized.

In the example of the method for interpolating moving images in thisembodiment, as illustrated in FIG. 43A, a generation image which isgenerated on the basis of two input images that are temporally adjacentis displayed in a period after one image is displayed until the otherimage is displayed, so that moving image interpolation can be performed.In this case, a display cycle of a display image is preferably half ofan input cycle of the input image. Note that the display cycle is notlimited to this and can be a variety of display cycles. For example,when the length of the display cycle is smaller than half of that of theinput cycle, moving images can be displayed more smoothly.Alternatively, when the length of the display cycle is larger than halfof that of the input cycle, power consumption can be reduced. Note thathere, an image is generated on the basis of two input images which aretemporally adjacent; however, the number of input images to be used isnot limited to two and can be other numbers. For example, when an imageis generated on the basis of three (or more than three) input imageswhich are temporally adjacent, a generation image with higher accuracycan be obtained as compared to the case where an image is generated onthe basis of two input images. Note that the display timing of the image5121 is the same as the input timing of the image 5122, that is, thedisplay timing is one frame later than the input timing. However, thedisplay timing in the method for interpolating moving images in thisembodiment is not limited to this and can be a variety of displaytimings. For example, the display timing can be delayed with respect tothe input timing by more than one frame. Thus, the display timing of theimage 5123 which is the generation image can be delayed, which allowsenough time to generate the image 5123 and leads to reduction in powerconsumption and manufacturing cost. Note that when the display timing issignificantly delayed with respect to the input timing, a period forholding an input image becomes longer, and the memory capacity forholding the input image is increased. Therefore, the display timing ispreferably delayed with respect to the input timing by approximately oneto two frames.

Here, an example of a specific generation method of the image 5123,which is generated on the basis of the images 5121 and 5122, isdescribed. It is necessary to detect motion of an input image in orderto interpolate moving images. In this embodiment, a method called ablock matching method can be used in order to detect motion of an inputimage. Note that this embodiment is not limited to this, and a varietyof methods (e.g., a method for obtaining a difference of image data or amethod using Fourier transformation) can be used. In the block matchingmethod, first, image data for one input image (here, image data of theimage 5121) is stored in a data storage means (e.g., a memory circuitsuch as a semiconductor memory or a RAM). Then, an image in the nextframe (here, the image 5122) is divided into a plurality of regions.Note that the divided regions can have the same rectangular shapes asillustrated in FIG. 43A; however, the divided regions are not limited tothem and can have a variety of shapes (e.g., the shape or size variesdepending on images). After that, in each divided region, data iscompared to the image data in the previous frame (here, the image dataof the image 5121), which is stored in the data storage means, so that aregion where the image data is similar to each other is searched. FIG.43A illustrates an example in which the image 5121 is searched for aregion where data is similar to that of a region 5124 in the image 5122,and a region 5126 is found. Note that a search range is preferablylimited when the image 5121 is searched. In the example of FIG. 43A, aregion 5125 which is approximately four times as large as the region5124 is set as the search range. By making the search range larger thanthis, detection accuracy can be increased even in a moving image withhigh-speed motion. Note that search in an excessively wide range needsan enormous amount of time, which makes it difficult to realizedetection of motion. Thus, the region 5125 is preferably approximatelytwo to six times as large as the area of the region 5124. After that, adifference of the position between the searched region 5126 and theregion 5124 in the image 5122 is obtained as a motion vector 5127. Themotion vector 5127 represents motion of image data in the region 5124 inone frame period. Then, in order to generate an image showing theintermediate state of motion, an image generation vector 5128 obtainedby changing the size of the motion vector without change in thedirection thereof is generated, and image data included in the region5126 of the image 5121 is moved in accordance with the image generationvector 5128, so that image data in a region 5129 of the image 5123 isgenerated. By performing a series of processings on the entire region ofthe image 5122, the image 5123 can be generated. Then, by sequentiallydisplaying the input image 5121, the generation image 5123, and theinput image 5122, moving images can be interpolated. Note that theposition of an object 5130 in the image is different (i.e., the objectis moved) between the images 5121 and 5122. In the generated image 5123,the object is located at the midpoint between the object in the image5121 and the object in the image 5122. By displaying such images, motionof moving images can be made smooth, and blur of moving images due toafterimages or the like can be reduced.

Note that the size of the image generation vector 5128 can be determinedin accordance with the display timing of the image 5123. In the exampleof FIG. 43A, since the display timing of the image 5123 is the midpoint(½) between the display timings of the images 5121 and 5122, the size ofthe image generation vector 5128 is half of that of the motion vector5127. Alternatively, for example, when the display timing is ⅓ betweenthe display timings of the images 5121 and 5122, the size of the imagegeneration vector 5128 can be ⅓, and when the display timing is ⅔between the display timings of the images 5121 and 5122, the size of theimage generation vector 5128 can be ⅔.

Note that in the case where a new image is generated by moving aplurality of regions having different motion vectors in this manner, aportion where one region has already been moved to a region that is adestination for another region or a portion to which any region is notmoved is generated in some cases (i.e., overlap or blank occurs in somecases). For such portions, data can be compensated. As a method forcompensating an overlap portion, a method by which overlap data isaveraged; a method by which data is arranged in order of priorityaccording to the direction of motion vectors or the like, andhigh-priority data is used as data in a generation image; or a method bywhich one of color and brightness is arranged in order of priority andthe other thereof is averaged can be used, for example. As a method forcompensating a blank portion, a method by which image data of theportion of the image 5121 or the image 5122 is used as data in ageneration image without modification, a method by which image data ofthe portion of the image 5121 or the image 5122 is averaged, or the likecan be used. Then, the generated image 5123 is displayed at the timingin accordance with the size of the image generation vector 5128, so thatmotion of moving images can be made smooth, and the decrease in qualityof moving images because of afterimages or the like due to hold drivingcan be suppressed.

In another example of the method for interpolating moving images in thisembodiment, as illustrated in FIG. 43B, when a generation image which isgenerated on the basis of two input images that are temporally adjacentis displayed in a period after one image is displayed until the otherimage is displayed, each display image is divided into a plurality ofsubimages to be displayed. Thus, moving images can be interpolated. Thiscase can have advantages of displaying a dark image at regular intervals(advantages of making a display method closer to impulsive display) inaddition to advantages of a shorter image display cycle. In other words,blur of moving images due to afterimages or the like can be furtherreduced as compared to the case where the length of the image displaycycle is just made to half of that of the image input cycle. In theexample of FIG. 43B, “input” and “generation” can be similar to theprocessing in the example of FIG. 43A; therefore, the descriptionthereof is not repeated. For “display” in the example of FIG. 43B, oneinput image and/or one generation image can be divided into a pluralityof subimages to be displayed. Specifically, as illustrated in FIG. 43B,the image 5121 is divided into subimages 5121 a and 5121 b and thesubimages 5121 a and 5121 b are sequentially displayed so as to makehuman eyes perceive that the image 5121 is displayed; the image 5123 isdivided into subimages 5123 a and 5123 b and the subimages 5123 a and5123 b are sequentially displayed so as to make human eyes perceive thatthe image 5123 is displayed; and the image 5122 is divided intosubimages 5122 a and 5122 b and the subimages 5122 a and 5122 b aresequentially displayed so as to make human eyes perceive that the image5122 is displayed. That is, the display method can be made closer toimpulsive display while the images perceived by human eyes are similarto those in the example of FIG. 43A, so that blur of moving images dueto afterimages or the like can be further reduced. Note that the numberof division of subimages is two in FIG. 43B; however, the number ofdivision of subimages is not limited to this and can be other numbers.Note that subimages are displayed at regular intervals (½) in FIG. 43B;however, timing of displaying subimages is not limited to this and canbe a variety of timings. For example, when timing of displaying darksubimages (5121 b, 5122 b, and 5123 b) is made earlier (specifically,timing at ¼ to ½), the display method can be made much closer toimpulsive display, so that blur of moving images due to afterimages orthe like can be further reduced. Alternatively, when the timing ofdisplaying the dark subimages is delayed (specifically, timing at ½ to¾), the length of a period for displaying a bright image can beincreased, so that the display efficiency can be increased and powerconsumption can be reduced.

Another example of the method for interpolating moving images in thisembodiment is an example in which the shape of an object which is movedin an image is detected and different processings are performeddepending on the shape of the moving object. FIG. 43C shows displaytiming as in the example of FIG. 43B and illustrates the case wheremoving letters (also referred to as scrolling texts, subtitles,captions, or the like) are displayed. Note that since “input” and“generation” may be similar to those in FIG. 43B, they are notillustrated in FIG. 43C. The amount of blur of moving images by holddriving varies depending on properties of a moving object in some cases.In particular, blur is often recognized remarkably when letters aremoved. This is because eyes track moving letters to read the letters, sothat hold blur is likely to occur. Further, since letters often haveclear outlines, blur due to hold blur is further emphasized in somecases. That is, determining whether an object which is moved in an imageis a letter and performing special processing when the object is theletter are effective in reducing hold blur. Specifically, when edgedetection, pattern detection, and/or the like are/is performed on anobject which is moved in an image and the object is determined to be aletter, motion compensation is performed even on subimages generated bydivision of one image so that an intermediate state of motion isdisplayed. Thus, motion can be made smooth. In the case where the objectis determined not to be a letter, when subimages are generated bydivision of one image as illustrated in FIG. 43B, the subimages can bedisplayed without change in the position of the moving object. FIG. 43Cillustrates the example in which a region 5131 which is determined to beletters is moved upward, and the position of the region 5131 isdifferent between the images 5121 a and 5121 b. Similarly, the positionof the region 5131 is different between the images 5123 a and 5123 b,and between the images 5122 a and 5122 b. Accordingly, motion of lettersfor which hold blur is particularly easily recognized can be madesmoother than that by normal motion compensation frame rate doubling, sothat blur of moving images due to afterimages or the like can be furtherreduced.

Embodiment 12

The semiconductor device can be applied to a variety of electronicdevices (including amusement machines). Examples of electronic devicesare television devices (also referred to as televisions or televisionreceivers), monitors for computers and the like, cameras such as digitalcameras and digital video cameras, digital photo frames, mobile phonedevices (also referred to as mobile phones or cellular phones), portablegame machines, portable information terminals, sound reproducingdevices, and large game machines such as pachinko machines.

FIG. 31A illustrates an example of a television device 9600. In thetelevision device 9600, a display portion 9603 is incorporated into ahousing 9601. The display portion 9603 can display an image. Further,the housing 9601 is supported by a stand 9605 here.

The television device 9600 can be operated with an operation switch ofthe housing 9601 or a separate remote controller 9610. With an operationkey 9609 of the remote controller 9610, channels and volume can becontrolled and an image displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the receiver, general television broadcast canbe received. Further, when the television device 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 31B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated into a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displaydata of an image taken with a digital camera or the like and function asa normal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (e.g., a USB terminal or aterminal which can be connected to various cables such as a USB cable),a recording medium insertion portion, and the like. Although thesecomponents may be provided on the surface on which the display portionis provided, it is preferable to provide them on the side surface or therear surface for the design of the digital photo frame 9700. Forexample, a memory storing data of an image taken with a digital camerais inserted in the recording medium insertion portion of the digitalphoto frame, and the image data can be transferred and then displayed onthe display portion 9703.

Further, the digital photo frame 9700 may be configured to transmit andreceive data wirelessly. The structure may be employed in which desiredimage data is transferred wirelessly to be displayed.

FIG. 32A is a portable game machine including two housings of a housing9881 and a housing 9891. The housings 9881 and 9891 are connected with ajoint portion 9893 so that the portable game machine can be opened andfolded. A display portion 9882 is incorporated into the housing 9881,and a display portion 9883 is incorporated into the housing 9891.Moreover, the portable game machine illustrated in FIG. 32A is providedwith a speaker portion 9884, a recording medium insertion portion 9886,an LED lamp 9890, input means (operation keys 9885, a connectionterminal 9887, a sensor 9888 (having a function of measuring force,displacement, position, speed, acceleration, angular velocity, rotationnumber, distance, light, liquid, magnetism, temperature, chemicalsubstance, sound, time, hardness, electric field, current, voltage,electric power, radial ray, flow rate, humidity, gradient, vibration,odor, or infrared ray), and a microphone 9889), and the like. It isneedless to say that the structure of the portable game machine is notlimited to that described above. The portable game machine can have astructure in which additional accessory equipment is provided asappropriate as long as at least the semiconductor device is provided.The portable game machine in FIG. 32A has a function of reading aprogram or data stored in a recording medium to display it on thedisplay portion, and a function of sharing information with anotherportable game machine by wireless communication. Note that a function ofthe portable game machine in FIG. 32A is not limited to those describedabove, and the portable game machine can have a variety of functions.

FIG. 32B illustrates an example of a slot machine 9900, which is a largeamusement machine. In the slot machine 9900, a display portion 9903 isincorporated into a housing 9901. Moreover, the slot machine 9900 isprovided with operation means such as a start lever and a stop switch, acoin slot, a speaker, and the like. Needless to say, the structure ofthe slot machine 9900 is not limited to the above structure. The slotmachine can have a structure in which additional accessory equipment isprovided as appropriate as long as at least the semiconductor device isprovided.

FIG. 33A illustrates an example of a mobile phone 1000. The mobile phone1000 is provided with a display portion 1002 incorporated into a housing1001, an operation button 1003, an external connection port 1004, aspeaker 1005, a microphone 1006, and the like.

When the display portion 1002 of the mobile phone 1000 illustrated inFIG. 33A is touched with a finger or the like, data can be input intothe mobile phone 1000. Further, operation such as making calls andtexting can be performed by touching the display portion 1002 with afinger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode, which is a combination of the twomodes, that is, a combination of the display mode and the input mode.

For example, in the case of making a call or texting, a text input modemainly for inputting text is selected for the display portion 1002 sothat letters displayed on a screen can be input. In that case, it ispreferable to display a keyboard or number buttons on most of the screenof the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1000, display on the screen of the display portion 1002 canbe automatically changed by determining the orientation of the mobilephone 1000 (whether the mobile phone 1000 stands upright or is laid downon its side).

The screen modes are changed by touching the display portion 1002 orusing the operation buttons 1003 of the housing 1001. Alternatively, thescreen modes may be changed depending on the kind of image displayed onthe display portion 1002. For example, when a signal of an imagedisplayed on the display portion is data of moving images, the screenmode is changed to the display mode. When the signal is text data, thescreen mode is changed to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 1002 is detected, the screen modemay be controlled so as to be changed from the input mode to the displaymode.

The display portion 1002 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenwhen the display portion 1002 is touched with a palm or a finger,whereby personal identification can be performed. Further, when abacklight which emits near-infrared light or a sensing light sourcewhich emits near-infrared light is provided in the display portion, animage of a finger vein, a palm vein, or the like can be taken.

FIG. 33B illustrates another example of a mobile phone. The mobile phonein FIG. 33B includes a display device 9410 in a housing 9411, whichincludes a display portion 9412 and operation buttons 9413; and acommunication device 9400 in a housing 9401, which includes manualoperation buttons 9402, an external input terminal 9403, a microphone9404, a speaker 9405, and a light-emitting portion 9406 that emits lightwhen receiving a call. The display device 9410 having a display functioncan be detached from and attached to the communication device 9400having a telephone function in two directions shown by arrows.Accordingly, short axes of the display device 9410 and the communicationdevice 9400 can be attached to each other, or long axes of the displaydevice 9410 and the communication device 9400 can be attached to eachother. Further, when only a display function is necessary, the displaydevice 9410 may be detached from the communication device 9400 so thatthe semiconductor device 9410 can be used by itself. The communicationdevice 9400 and the display device 9410 can transmit and receive imagesor input information to/from each other by wireless communication orwired communication, and each of the communication device 9400 and thedisplay device 9410 has a rechargeable battery.

Note that this embodiment can be implemented in combination with any ofthe other embodiments as appropriate.

This application is based on Japanese Patent Application serial no.2009-051857 filed with Japan Patent Office on Mar. 5, 2009, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising: forming a first electrode, a second electrode and asemiconductor layer over an insulating surface so that each of the firstelectrode and the second electrode is electrically connected to thesemiconductor layer, each of the first electrode and the secondelectrode comprising a first conductive layer and a second conductivelayer, forming an insulating layer over the semiconductor layer, forminga third electrode over the insulating layer, the third electrodecomprising a third conductive layer having a light-transmitting propertyand a fourth conductive layer.
 2. The method for manufacturing thesemiconductor device according to claim 1, wherein the semiconductorlayer comprises an oxide semiconductor containing indium, gallium, andzinc.
 3. A semiconductor device comprising: a substrate having aninsulating surface; a first electrode and a second electrode over theinsulating surface, each of the first electrode and the second electrodecomprising a first conductive layer and a second conductive layer; asemiconductor layer that has a light-transmitting property and iselectrically connected to the first electrode and the second electrode;an insulating layer over the semiconductor layer; a third electrode overthe insulating layer, the third electrode comprising a third conductivelayer having a light-transmitting property and a fourth conductivelayer.
 4. The semiconductor device according to claim 3, wherein thefirst conductive layer has a light-transmitting property.
 5. Thesemiconductor device according to claim 3, wherein the insulating layeris a gate insulating layer between the semiconductor layer and the thirdelectrode.
 6. The semiconductor device according to claim 3, wherein thefourth conductive layer is over the third conductive layer.
 7. Thesemiconductor device according to claim 3, wherein a lower surface ofthe semiconductor layer is in contact with the first electrode and thesecond electrode.
 8. The semiconductor device according to claim 3,wherein the third electrode is a gate electrode, and wherein the numberof the gate electrode is one.
 9. The semiconductor device according toclaim 3, wherein the semiconductor device comprises a pixel portion anda driving circuit portion, and wherein the driving circuit portioncomprises the semiconductor layer.
 10. The semiconductor deviceaccording to claim 3, wherein the semiconductor layer comprises an oxidesemiconductor containing indium, gallium, and zinc.
 11. Thesemiconductor device according to claim 3, further comprising: a lightemitting element, wherein the light emitting element is electricallyconnected to the semiconductor layer.
 12. A television comprising: asemiconductor device comprising: a substrate having an insulatingsurface; a first electrode and a second electrode over the insulatingsurface, each of the first electrode and the second electrode comprisinga first conductive layer and a second conductive layer; a semiconductorlayer that has a light-transmitting property and is electricallyconnected to the first electrode and the second electrode; an insulatinglayer over the semiconductor layer; a third electrode over theinsulating layer, the third electrode comprising a third conductivelayer having a light-transmitting property and a fourth conductivelayer.
 13. The television according to claim 12, wherein the firstconductive layer has a light-transmitting property.
 14. The televisionaccording to claim 12, wherein the insulating layer is a gate insulatinglayer between the semiconductor layer and the third electrode.
 15. Thetelevision according to claim 12, wherein the fourth conductive layer isover the third conductive layer.
 16. The television according to claim12, wherein a lower surface of the semiconductor layer is in contactwith the first electrode and the second electrode.
 17. The televisionaccording to claim 12, wherein the third electrode is a gate electrode,and wherein the number of the gate electrode is one.
 18. The televisionaccording to claim 12, wherein the semiconductor device comprises apixel portion and a driving circuit portion, and wherein the drivingcircuit portion comprises the semiconductor layer.
 19. The televisionaccording to claim 12, wherein the semiconductor layer comprises anoxide semiconductor containing indium, gallium, and zinc.
 20. Thetelevision according to claim 12, further comprising: a light emittingelement, wherein the light emitting element is electrically connected tothe semiconductor layer.